Pretty frequently..
Sometimes when it comes to arguements, I want to post my opinion. But after I finish writing all down, I stop, think, and go somewhere else and continue lurking xD;
And when it comes to posting something I wanted to show (usually my art), I change my mind a lot about it. I post the link, and then I remove it and think "nah, they wouldn't like it xD;;"
They are other reasons but those I said happen commonly. I'm more of a lurker xD; *brick'd*
But err yea >>; I often have second thoughts in real life too...