I typically stood up for other kids (to the point of interposing myself when someone was trying to bully them), but I did once stupidly play "keep away" with someone else's sneaker while he was crying and yelling for it back, right before one of the throws ended up losing the shoe in the pool.
About right then, my dad showed up, beat my butt, and that was the end of that.
Not sure why I did it. I remember being caught up in the moment. I think I also would have felt guilty afterwards, anyway, even if my dad had not walloped me good for it.
I was bullied a lot more myself, since I was in band and was the class smarty. I remember once when I had crutches in middle school, the other kids would take my crutch and pass it to the other end of the room and/or take off the rubber cap on one of them and do some XXX things with it and not give it back. If I didn't get the highest grades in class, people would make fun of me; and if I did get the highest grade, they'd also make fun of me.