I'd really like to know as well, since I'd like to travel eventually.
As for the canadian woman with improper paperwork, I can tell you usually the worst that can happen (well, before now, that is) was that they'd take you to a separate room for a few hours to check and question you, and, if things eventually check out, they let you through, else, you'd be sent back, especially at a land crossing. You wouldn't be detained for months, unless you committed some crime or had a warrant