18. Duncan is zealous about sticking to the "rules" in a challenge to the extent that even a second blade is verboten. [just watched "Valkerie" and was struck that he beheaded Ingrid without a fight, without formally challenging her. Also, he uses a second blade several times on the series (against Caspian, in Duende).]
19. (from darthhellokitty) Immortals are uncontrollably horny after a quickening. [this one is iffy, canon is unclear. I don't remember DM ever acting this way consistently if at all after a quickening. As pat_t pointed out in the previous discussion there are a couple of examples of this, one in the show and one in the stories by Gillian Horvath. I still think it can be called fanon, due to the frequency in which it crops up and the lack of compelling evidence in canon. I would say that quickenings are extremely intense, that in canon they are shown to have differing effects depending on the situation and the immortal beheaded. There are several times where DM is obviously profoundly effected - but he's mostly shown reacting by doing intense katas. Perhaps he's working off his uncontrollable sexual urge, but that's not clear from canon.]
20. Can't believe I forgot this: DM and Methos have some sort of psychic bond after the double quickening in Bordeaux. [I actually like this one, and enjoy many stories featuring it. But there's nothing in canon to indicate it. It's more like a brilliant idea someone came up with in a fic early on that has gotten passed down nearly universally. I've used it myself.]