I had this happen to me. When I entered Canada (Quebec) from the US the immigration agent asked me the purpose of my visit. I told him that I was just transiting to catch a flight at YUL (Trudeau). So he gave me a transit visa along with some kind of form that I was supposed to give to immigration at the airport before departing. Seemed easy enough to me so I didn't resist the idea. After, all I really was transiting. Why lie about it? Well, I had an early morning flight and the YUL immigration office was closed. There was no immigration at any point before my flight and of course there was no way for me to get an exit stamp. So as far as Canadian immigration is concerned I have overstayed my visa by about 10 years at this point. I've talked with Canadian immigration a few times to try to correct the issue, but the problem is I lack proof. For all I know I may never be able to enter Canada again. It's too bad because I like Canada better than the US.