10 years ago I was working in Canada; couple of friends and I (Australian, British and Québécois) decided to go and ski in Montana for a few days. We had a few beers on the way down and stopped just before customs to drop off open cans before we crossed the border. Being 11pm, we were the only people at the crossing. As we circled round they decided something wasn't right (probably justified although not in their jurisdiction) - 4 hours later we were allowed into the US having been fingerprinted and our car searched on a ramp for what I assume was explosives or drugs.
3 days later we returned to the border travelling the other direction - the CBSA officer looked at the cover of all three different nations' passports before saying "I'm sure there's a visa in there somewhere, have a nice day."