Most of the stupid things I did in a forum related to smileys and RPGs. Back before I joined these forums, I was mainly on some anime-based forum, and I used smileys a LOT. Although, on that forum it was pretty easy to do, as they had a lot of really elaborate/humorous smileys. I especially used them in the RPG forum, so when I joined these forums, which discourage the use of smilies in the RPG forum, I had to make serious efforts to cut back on them.
Speaking of the RPG forum, the previous forum I was on came out with an RPG section one day. I didn't know anything about play-by-post RPGs at the time; I looked at all the threads there and they seemed to be a bunch of people collaborating to write a story. So I posted in one of the story threads and had a character or two of my own interact with the others, and got a lot of weird reactions from the participants. They also had a separate forum for sparring; I once got in trouble for posting in a sparring thread and jumping into the fight without permission. Served me right for not reading the rules first...
On these forums, the only stupid thing I can recall doing was also in the RPG forum; I signed up for an RPG with specific roles to fill. Someone else had already taken the role I wanted, but I signed up for it anyway. The GM didn't get mad at me, but he did have us each do writing samples as a sort of contest to see who would get the spot. I wrote mine first, and the other person competing for the spot then wrote something that was clearly much better than what I wrote. I felt like such a jerk afterwards...
Oh, and I also remember posting a thread in the Miscellaneous forum (this was on the old forum, not here) that basically had me complaining about how nobody liked me and how stupid I was, accompanied by lots of crying smilies. Needless to say, the thread got closed pretty quickly.