I see bad data and thought to myself, "I'll be able to fix this with a simple regex." A month later, I'm still finding new data patterns. Never give users the ability to store all of their data in a huge textarea box where they can make stuff up.

"Shit in.." etc.

Rule of thumb:

User = brainless fucking feces-throwing monkey who ruins absolutely everything, all the time.

But at the same time, you are a user, too. Don’t step on their toes right limit user ability because that’s just being a douchecanoe; instead, make the error correction solid and the UX both idiot and asshole proof.