Canada US Mixed Franking

Here's one I can't figure out. A cover franked with both US and Canadian postage, with a Montreal hotel return address, but a Detroit cancellation and Wisconsin back stamp. How did it get to Detroit without a Canadian postmark?

It might be as simple as a letter written in Canada, ready for posting. And then comes that moment back in Detroit when you see you fogot to post the letter.

That makes sense, I guess. The sender just wrote off the Canadian 3¢ stamp.

If the sender was a resident of the US and had visited Montreal either on business or vacation, the Canadian stamp would have no value to him after returning home.

Especially with tourist-mail, it is very common to see US/Canadian mail franked in one country, then carried across the border and re-franked with stamps from the other country before mailing.

And equally common to go ahead and mail the letter with only the incorrect country's stamp and see if it gets caught or goes through!

There have been one or two threads on this topic in recent months. US/Canada seem to be the most common one found.