Hmm. Probably a toss-up between:
-Attempting to race one of my friends down a really short road that quickly merged two lanes into one. I tried to pass him and wasn't fast enough, so we were right next to each other when the lanes merged. I had to slam on my breaks not to hit him, ended up spinning out and hitting the curb. Thankfully nobody besides my friends and I saw the event. It could've been a lot worse.
-I once punched right through a window. Not a REAL window, mind...just one of the panels on a glass door. Really thin. Believe me, I was just as surprised as anyone. Fortunately no real damage was done, just a nasty cut on my wrist and a whole lot of embarrassment. Which brings us to the "why" of the matter...I hit my shoulder on the door on my way through it, so I punched it. Didn't even think about it. It was like a reflex. I wasn't even remotely angry until I hit my shoulder. In fact, I was in a pretty good mood. Then I just...punched it. I've always had some pretty serious anger management issues, but this was...probably the worst instance of it. (Pro tip: Do NOT play videogames while angry. Especially not Shadow of the Colossus. And ESPECIALLY not while your grandmother is in the house. I'll just leave it that.)
-I once made a joke about cancer. Now, I was a kid at the time, but I knew what I was doing. What's that you say? You think it's not that bad? Did I forget to mention that my friend, who's mother had severe leukemia (a fact I was well aware of at the time) was in the room? I mean, I wasn't trying to offend him or anything, I just didn't think. Still, I literally slapped myself several times after that. Still feel awful about it.
Now if you'll excuse me, I have to go try to slap myself so hard that my idiot past self feels the pain. That moron has it coming.