One-a Them "A" Countries
Americans confusing Australia for Austria is a joke that just never gets old. As it turns out, now you can get the same laffs online, without even meeting an American!
There's even a map, so you can see just how obviously wrong they are.
Tracing the history of the Austr(ia,alia) confusion involves more Google mojo than I have at my command, but I did discover this rather comprehensive history of common memes on the alt.religion.kibology newsgroup, which finds a 1994 reference to a troll describing Kylie Minogue as an Austrian. But I'm sure the joke has been around for longer than that.