First account: posting in the Dear Adult Swim thread two different times in the same night with a spoiler-filled question in the hopes they'd answer immediately after the offending episode's TV broadcast.
Second account: lying about being over 18 so I could use the private messaging function, which was necessary for a Death Note fanfic character submission happening on the boards. If I had known my character would be killed off after three chapters of their being present, I wouldn't have bothered.
Third account: linking to a site containing hentai imagery (without knowing there was hentai on that side) because I was worried an article I read on another website I knew was not safe for linking on the boards was right in that my computer could be remotely turned into a bomb.
Learning experiences.