I don't believe there is ANY situation at a border in which you cannot be denied entry -- border officers have immense discretion. If you are trying to ask if you are guaranteed re-entry to Canada, the answer is no.
The only situation I can think of where they might not be permitted to refuse you is if you are requesting asylum -- since that assumes that your life is in danger, you are not supposed to be turned back. Even that, though, might not work on the U.S. border because of the general resistance to accepting refugees or asylum-seekers from stable, developed countries.