Only one. Getting too personal/sexual in a private message until it got to the point that the guy reverse tracked my profile picture and found me on facebook, got my cell number and started sending inappropriate pictures and messages and threatening to tell my boyfriend if I didn't reciprocate. I had already told my boyfriend and apologized profusely because I felt extremely guilty about it, it's the one thing I wish I could take back. He also figured out where I lived and I got a random text from him a few months later saying to meet him at a bar about 5 minutes from my house. He was creepy and I blocked his number, blocked him on GAG, and made my facebook profile private.
I thought the greatest mistake I made was when I responded to a vaccination thread and was pointed out to be an idiot.
In actuality, the greatest mistake I made was not responding to a thread/blog/post when it was up, and it either disappeared or became buried with new crap. (If I recall correctly, I did not have an email and could not actually sign in at the time...)