My number one pet peeve is when people come into a thread, state something as though it's the truth and when questioned we find out that their second cousin on their mother's side's sister's boyfriend was drunk one night and told them this was a fact and they accepted it as so and now share it with others.

In short... stating misinformation as fact.

My other pet peeve... people who believe whatever they read without understanding that GOOGLE IS THEIR FRIEND and they can (and should) check it out before believing what they just read!