I think I've done something like this before, who knows when... Let's do it again, anyway. ;/
(There's a possibility for spoilers, so don't read the plot device if you see a series that you haven't seen yet and would not like anything spoiled!)
Of course, this one's right off my bat. Perhaps again. Its horror haunts me till this day.
Anime/manga series: Fushigi Yugi
Plot device: Yui, a virgin, is told that she was raped (while she was unconscious). And... Somehow believed it despite there being no evidence of a rape. I don't think it takes a genius (and she's supposed to be a smart girl) to know that some thing's different down there or not, even (or perhaps especially) when a girl is a virgin. The problem with this is that the majority of the story wouldn't have happened had she realized the truth in the beginning. And, honestly, I really don't see how she couldn't have realized it... Seriously. No way. Or, at least, not in her case.
Okay, it's your turn. ;/