I escalated access to my school district's webserver and accidentally took it down. For some reason I was trying to close the hole I had used; instead, I default denied all access. Whoops. I explained what happened to a teacher, he called in the IT group, my change was reverted and nothing was ever spoken about it again.

I also hacked the online grade review site, which was trivially hackable (javascript password protection, wtf), told a teacher, nothing happened to fix it (or to damage me) and I dropped the issue.

Both of those were done in attempt to be altruistic but technically both were completely illegal. Since no administrator ever found out, no cops were ever called and no prosecutorial discretion was in play, but I wouldn't want to bet I'd have been "let off", which is sort of terrifying.