I've never done anything I truly consider significantly bad, but there are a lot of things, however little, that I regret.
-Two guys in front of me in an assembly were pointing at some girls and saying "She's a ho. She's a ho. She's a ho." and I muttered out "All the girls are hoes, and all the guys are their pimps." The two guys took me literally and started telling everyone I'd said that, and the nearby girls got pissed. I was joking, and I doubt anyone but me remembers it, but I still beat myself up for this.
-A similar example; two guys were biking down the street as I was walking home, and the other guys walking home (we had gotten off the bus) were commenting about the two being on the same bike, and I hummed "a bicycle built for two" to myself, and then one of the guys was sorta riding on my heels. I told him I didn't mean it, but again, I don't feel right about it.
-In an online journal I had written little notes about classmates and my personal opinions of some of them (I hadn't finished, and never did). About a year later, long after I had stopped caring about the thing, some classmates found it googling themselves and were telling me how weird and creepy it was. I actually had let my cousin operate the account for the past year or so, and had to ask him to take the postings down.
There are numerous that come to mind on specific occasions, just little things where I could've handled the situation better. If I think of anything more substantial, I'll bring it up.