I don't say any bad words or swears, but sometimes I say quite mean things unintentionally like this:

Me: You two remind me of these two guys Ryan and Paul from our lounge, especially since you have the exact same names as them and you kind of look like then.
Some Other Guy that isn't Ryan or Paul: Yeah, but our Paul is black.
Me: Well this Paul is kind of black. No offense.

Even though I said no offense to be nice about it, everyone started laughing about how offensive it sounded.