Even the President thinks so! In his talk yesterday, he warned kids about Facebook, and being careful what they should post. It’s also what I talk about in my branding and blogging talks, be careful what you post.
However it’s not a permanent thing you can’t get over. At least in the US we are likely to give you a second chance if you post something in appropriate.
Each post, however, adds to your reputation. If you have too many things that show you’re not very responsible, it looks bad.