What about the infamous
Alyson Hannigan Sex Video? (NSFW, obviously)
OK, it isn't her. To quote from the page, "When you think about it, the naïveté of some porn fans is kind of cute: we love the fact that there are folks out there who love “Buffy“‘s Alyson Hannigan enough to suspend their better judgement and believe that the redheaded model in this hardcore hotel room clip is actually her. (Besides, thinking about it that way makes us less pissed off about all the sleazy smut peddlers who try to pass this stuff off as real.)"