I have a story, though it may not be as funny as some, and is only a slight bit of ignorance, but it still seemed pretty funny at the time.
I have a friend who was dating this girl once. First girl he actually met in person, and said she was a freak in bed. So obviously, I sat with wide-eye anticipation at what stories he was going to tell... *skips to the only slightly ignorant part* ...He told me she has had sex with several different guys, and was a necrophiliac. I just stared at him, with a kind of confused, horrified face, till he asked what. I than said, "She has sex with dead bodied!?" He then looked at me confused, and said he never said that. I pointed out he said necrophiliac, and he said yeah, a person that has alot of sexual intercourse... I then corrected him by saying that necrophilia was the act of have sex with a deceased body, Nymphomania was have lots of sex. He then faced palmed before telling me that he already told some guys on XBL that his new GF was a necro...
I managed to prevent myself from laughing at his misfortune until I left his place...