Chuck, it's be easy enough for a card to be mislaid and put into a bag of mail for Malta. When they found it, the Maltese PO would probably just postmark it and send it - even if they bothered to notice the stamp wasn't their own.
I work in a British Post Office and we had a problem with people sending stuff to South Australia without writing Australia at the bottom. The thing is that with international mail only the last line is looked at, so it can end up in the wrong country quite easily. With the South Australia thing the post was ending up in South Africa, who very kindly sent it on, but I'm sure that it would have a South African postmark on it somewhere.