Monsters can get stuck in a weird state if you transition levels while they are in the middle of charging. They get stuck as sort of missiles, sort of monsters, where their graphics don't load, but they occupy a tile, and can be moused over to show their information. Click them will launch an attack (with a bow equipped, with melee your character will walk up if not close and then attack). You cannot harm the monster in any way with it in this state (i.e. all melee attacks fail, ranged attacks go by, and firewall does nothing). The only thing that fixes the monster is to use telekinesis on it, but this only works if the tile to which telekinesis will knock it back is open; if not, the monster is forever bugged.

@NiteKat do you have a save game to reproduce? Would be very useful for trying to fix the issue.

Monsters can get stuck in a weird state if you transition levels while they are in the middle of charging. They get stuck as sort of missiles, sort of monsters, where their graphics don't load, but they occupy a tile, and can be moused over to show their information. Click them will launch an attack (with a bow equipped, with melee your character will walk up if not close and then attack). You cannot harm the monster in any way with it in this state (i.e. all melee attacks fail, ranged attacks go by, and firewall does nothing). The only thing that fixes the monster is to use telekinesis on it, but this only works if the tile to which telekinesis will knock it back is open; if not, the monster is forever bugged.

@NiteKat do you have a save game to reproduce? Would be very useful for trying to fix the issue.

diasurgical/devilutionX#2879
it's trivial to reproduce and trivial to fix ;) nothing interesting

Monsters can get stuck in a weird state if you transition levels while they are in the middle of charging. They get stuck as sort of missiles, sort of monsters, where their graphics don't load, but they occupy a tile, and can be moused over to show their information. Click them will launch an attack (with a bow equipped, with melee your character will walk up if not close and then attack). You cannot harm the monster in any way with it in this state (i.e. all melee attacks fail, ranged attacks go by, and firewall does nothing). The only thing that fixes the monster is to use telekinesis on it, but this only works if the tile to which telekinesis will knock it back is open; if not, the monster is forever bugged.

@NiteKat do you have a save game to reproduce? Would be very useful for trying to fix the issue.

diasurgical/devilutionX#2879
it's trivial to reproduce and trivial to fix ;) nothing interesting

Somehow this had just never been documented here.

Never seen this mentioned as a bug, so here's my take. There are two unique items in the game which can spawn on two base items. "Gnarled Root" (Club/Spiked Club) and "Stormshield" (Tower/Gothic Shield). This is definitely a bug as items in the global item table are supposed to have a unique ID per item, so unique items can reference that ID. However, Club/Spiked Club use the same ID instead of the latter being none. Same applies to tower shield.

Additionally, Warrior's Club has the same entry for unique item lookup as Club. It should be empty however. Lightforge also uses the wrong lookup type in the unique item table, using a regular mace, allowing it to drop. It always morphs on a new game because of this. It is a quest item and special drop.

Empty mana orb doesn't match background:

(there are bits of gold in the wing from a previous UI element at that position)

The way diablo chooses levels for magic affixes is buggy and leads to several problems. So I'll explain. First they are chosen in one of three ways depending on item type:

• All items but staves: min level is 1/2 base level and capped to 25
• Staves with charges: no minimum affix level
• Staves without charges: min level is 1/2 base level and not capped

Bug # 1: Staves are like jewelry in that they are always supposed to be magic or contain charges. It is possible however to find a plain white staff. For example, a level 4 monster drops a magic staff with a suffix, however there are no suffixes between the level range (2-4) thus it drops without anything.

Bug # 2: Minimum magic level is capped to 25 max. Now only Wirt is capable of generating items this high. Why is this a problem? It means past a certain point some affixes stop spawning. At character level 50, Wirt can no longer sell Obsidian (thus the famous Ob Zods).

When fixing monsters to drop with their difficulty bonus, it opens up a plethora of affix bugs.

• Level 50+ monsters can't drop staves with a suffix
• Various other items have few/no affixes once monsters reach a high enough level

When removing the level cap, it additionally opens up these bugs:

• Rings/amulets dropped by level 56+ monsters can only spawn with one affix, Zodiac. No prefixes spawn
• Certain unique monsters such as Lazarus, on hell difficulty, will only drop white rings/amulets (no affixes at all)

This may just be an odd design quirk, but once you fix monsters, the minimum level set on affixes really starts breaking the game.

Diablo's magical unique loot system explained using staff of shadows as an example:
Staff of shadows uses long staff (lvl 4) as a base and is lvl 2 itself.
If multiple unique items meet the requirements, the one lowest on the list gets chosen.
Immolator is lvl 4 and uses a long staff base too.

Which means in theory staff of shadows can never drop, because if lvl 4+ loot drops (to roll the long staff), then immolator will always gets picked over staff of shadows. Ofc you can get it to drop in single player if immolator drops first, because then it will stop blocking staff of shadows - such a staff will morph back into immolator in vanilla's new game.

However, it's possible to get a stable one because of a bug described in diasurgical/devilutionX#2763
Item generation is messed up because monster level increased by difficulty bonus/unique monster bonus is only used for generating the base item - affix/unique item generation uses monster's starting level (ignoring boss/difficulty modifiers)
Which means there's one scenario where staff of shadows can drop - lvl 2 or 3 monster on nightmare/hell difficulty.
Let's take a lvl 2 monster as an example.
Difficulty bonus = 15 on nightmare and 30 on hell.
Base item would roll with (2 + difficulty bonus) level, which means a long staff can drop.
But affix/unique item would get rolled with the original level = 2 = congrats, it's a staff of shadows!

To make it even more confusing, unique monsters have a hardcoded +4 bonus to affix/unique generation level BUT NOT the base item level - which means that you can only get staff of shadows from a NON-UNIQUE lvl 2-3 monster on nightmare/hell difficulty, because if it was a unique monster, the long staff could always hit immolator.

This is certainly a bug because there's 0 reason why for example celestial bow (which looks like something a lvl 3 rogue would be happy about but later? ...) can only drop on nightmare or hell ;)

To make it even more funny, it means that in singleplayer you can get some uniques in 2 versions - stable or unstable that will morph in a new game.
An unstable staff of shadows would drop if it only dropped because you already got immolator before it and a stable one would drop from a scenario described above.
Hope that's not very confusing, only took 25 years to find out 🤪

Adjacent attack damage from Monk, Bard, and Barbarian attacks with certain weapon types is handled by calling the damage function again but passing the negative pnum value for that player. The negative value signals to the function that the chancel to hit and damage penalties for adjacent damage should apply to the attack. However, player 1 has a pnum value of 0 so instead of a negative number, it just passes 0 and calculates damage as normal. This means the chance to hit and damage penalties do not apply to player 1, but they do apply to other players.

Runes of Lightning never deal damage. When a monster steps on the rune, a Lightball missile is generated on the tile, but damage is never set for the missile, so min and max damage are both 0. You'll hear it hit plenty of times, but every hit deals 0 damage.

Editing to add a thanks to @StephenCWills for looking into this one with me. Wasn't sure of his github handle at first so I didn't add it in the initial post. :)

https://github.com/diasurgical/devilutionX/blob/ba10fa31821e509058e77188dde77faf6815c2a6/Source/missiles.cpp#L1197

Because of how unshifted damage is randomized, 0-0 dmg will end up randomizing 0-63 (64 = 1 hp), so the damage is less than 1 hp, can add up - lightning rune had 0 dmg but ended up actually damaging the player because of this

Save data can overflow the save buffer. The buffer size is 262147 bytes, but a save can take up to 322586 bytes. In Hellfire the issue is more likely to happen as the save game can reach 327218, mostly because of additional item data.

diasurgical/devilutionX#3109

Monster tags (mWhoHit) for experience are not shared with players when they enter a level that has active players on it. This means that you can have a player receive less than full XP while another player receives full XP. This happens when a player has hit a monster, another player loads onto the level and kills the hit monster before anyone else hits the monster again. The player that loaded onto the floor only sees themselves hit the monster, so they get full XP, while the original player (or players) get split XP.

diasurgical/devilutionX#3110

I also noticed that monster tags get cleared (mWhoHit) if all players leave a floor. I'd say it's debatable if this is a bug or not, but feels like it is (and feels connected to the above issue).

Editing to add: diasurgical/devilutionX#3111

I did not test to confirm, but it is very likely this issue with mWhoHit is also Vanilla behavior. It's possible for mWhoHit to just desync altogether even if all players were on the floor during a fight with a monster. As described in the linked issue, the player that attempted the attack missed, but the other player saw the hit land, so mWhoHit desynced. If the player that saw a hit now kills the monster, they get half XP while the other player gets 0 XP. Monster health did not desync at all, so mWhoHit does not use the same mechanisms that damage to monsters use.

Monster hp is not synced when a player enters the level.
Player 1 damages a monster
Player 2 enters level and sees the monster with full hp.

Monsters will inform the group lader that they spotted the player, but the leader will not inform the rest of the pack. This can often lead to situations where groups are stuck because a single member of it is unaware that the fight has started and the rest of them do not want to leave him behind.

Scavengers may leave a group twice, once if separated, and a second time if they need to eat. This leads to the leaders pack size under flowing and the leader then thinking there are 255 minions who aren't following orders, preventing it from moving anymore during the game.

diasurgical/devilutionX#3158
Theme_Treasure - loop there reduces value of items and quantity of gold by half. It can hit the same item multiple times, so a gold pile can end up being 0.
Example log:

INFO: (36 84) GENERATING GOLD PILE: 54
INFO: (36 84) OVERWRITING GOLD PILE: 27
INFO: (36 84) OVERWRITING GOLD PILE: 13
INFO: (36 84) OVERWRITING GOLD PILE: 6
INFO: (36 84) OVERWRITING GOLD PILE: 3
INFO: (36 84) OVERWRITING GOLD PILE: 1
INFO: (36 84) OVERWRITING GOLD PILE: 0

The reason people don't really see this one is

1. It can only happen in catacombs and below
2. Treasure rooms are pretty rare
3. I guess this bug is pretty rare too
4. Nobody checked every single gold pile without alt highlighting.

This bug also has a second part, found by @galaxyhaxz
It reduces items' value by half, however that value gets restored to original on new game or identifying a magic item : D

Scavengers only search for corpses in south direction because of a bug - out of bounds check got applied to offset instead of final coords, so all negative offsets are considered invalid, only >= 0 pass = direction as on the pic :D

diasurgical/devilutionX#3260
#2260

If hit by a missile player can resist and had > 0 resistance against, the game would play the sound of attacker's class getting hit for both players.

Hellfire's hive ruined a tree in town :D you can still see a part of it and it got relocated

If you get killed by a barrel, death animation is a bit broken

diasurgical/devilutionX#3514
Can drop items inside houses if standing on the corner

diasurgical/devilutionX#3485
Pathing can end up to be broken near solid objects

For example here the character could pick the north gold pile just fine but moves northwest before doing so because of the river on right side.

Can drop items inside houses if standing on the corner

Good hiding place!

Algorithm for checking if you have the stats to use an equipped item is poopy.
diasurgical/devilutionX#3707

Items requiring no stats technically require 0 stats, so going negative makes you unable to use any items.
However the order matters - example situation with civerb's cudgel making barbarian reach negative magic.

You equip civerb's cudgel, your magic is -2.
The game checks first item which is helmet, sees it requires 0 stat (no stat requirement) or higher, sees that you have -2 magic, disables the helmet.
Then civerb's cudgel gets checked, it requires 0 magic, you have -2, civerb's gets disabled and penalty is cancelled so items after hand slots are usable!

Considering the slot order

INVLOC_HEAD,
INVLOC_RING_LEFT,
INVLOC_RING_RIGHT,
INVLOC_AMULET,
INVLOC_HAND_LEFT,
INVLOC_HAND_RIGHT,
INVLOC_CHEST,

it means that if you equipped a chest piece that brought your stats below 0, it'd make ALL items unusable : D

Like this, red rings and amulet, a rare sight : )

diasurgical/devilutionX#4113

void CheckInvSwap(int pnum, BYTE bLoc, int idx, WORD wCI, int seed, BOOL bId)
{
	PlayerStruct *p;

	RecreateItem(MAXITEMS, idx, wCI, seed, 0);

	p = &plr[pnum];
	p->HoldItem = item[MAXITEMS];

	if (bId) {
		p->HoldItem._iIdentified = TRUE;
	}

	if (bLoc < NUM_INVLOC) {
		p->InvBody[bLoc] = p->HoldItem;

		if (bLoc == INVLOC_HAND_LEFT && p->HoldItem._iLoc == ILOC_TWOHAND) {
			p->InvBody[INVLOC_HAND_RIGHT]._itype = ITYPE_NONE;
		} else if (bLoc == INVLOC_HAND_RIGHT && p->HoldItem._iLoc == ILOC_TWOHAND) {
			p->InvBody[INVLOC_HAND_LEFT]._itype = ITYPE_NONE;
		}
	}

	CalcPlrInv(pnum, TRUE);
}

This doesn't take barbarian's ability to wear 2h in 1h into account.
So that's how other players see you when you swap 1h + shield with a 2h. (shield disappears on smaller screen)

The fire for the torches in Lazarus' chamber is missing when opening the last room.