This would be totally unacceptable to me. I could understand if it was a billing error that she quickly resolved once it was brought to her attention. I could also understand if it was one of those critical sessions and she gave you the option of extending your session, with the understanding that it would mean an additional charge. BUT to just go over without discussing it and charge you for it !?!? That is not okay. ...having said that, I'd probably be way to scared to ever bring it up and would agonize about it for ages