In line with the replies above, a game I'd play with my friend which was kind of like playing chicken. There was a fairly steep service road that went from our neighborhood down to a major street, with a big ditch between. We'd ride down as quickly and far as possible on our bikes before applying the brakes, and one time I lost control and landed right near a bunch of broken beer bottles and stuff, although I ended up unscathed.
More stupid and damaging on a whole, pretty much any time I allowed my pride and/or moral beliefs to fuck myself over. Telling the truth when I should have lied, not getting even against a few that deserved it, doing things for others when I should have told them to fuck off just because I was afraid of being mean or committing a "sin". There isn't really a single event that stands out as the worst, but a typical one would be having a good friend that had moved away come by our school once to visit, seeing him hanging out with others and him looking over at, I assumed I was unworthy and left, years later in retrospect he probably thought I saw myself as the superior one.