I found a new twist recently. On checking in with Air Canada for a New York to Europe flight with my EU passport, that transited Montreal, they asked where my transit visa was! I said I never heard of such a thing. We went back and forth and then I asked if it would make a difference if I used my US passport. And it did. No transit visa required. Very surprising for me as I never had get a visa to visit Canada with a EU passport before. And I wasn't even entering Canada! It was just a transit.

I went back and looked at my ticket, which was an ex-EU round trip, and it says nothing about a transit visa in the fine print or anywhere else.