Every so often I end up at snopes.com to hear about some crazy email forward or urban legend. It's ok content you can often find elsewhere but they're the central place for much of it so it's easy to find them. In the past few months however, I noticed they added an obnoxious popup ad (fired in Flash I would presume, which I don't block) on top of all their articles. Today I noticed after following a link there that my back button no longer worked, redirecting me to the snopes page I was already on.
That's pretty awful thing to do to users -- forcing them to look at annoying flashing ads that jump out when their browser is set to avoid them and then not to let them leave.