In the past, I have seen petitions (especially on facebook) that people seem to sign without understanding the issues.  I can't think of one I haven't signed there.  One I did not sign because I was on the opposite side of the issue.  For another one, I wasn't strongly for the cause (if I had to vote I would have been on the side of the petition though).  I wouldn't sign the actual petition though, as it was very broad, had no defined statement, and was full of errors.

What do you consider when deciding whether or not to sign a petition?  Have you ever started one?