I run across this alot. I hear a song that got popular but it's popularity overshadows the original source. It's not really fair but anyway can anyone think of more examples? Here are two big ones:

I thought Johnny Cash originally did the song Rusty Cage, but to my surprise Soundgarden did it first.

When I was in middle school I had no idea that "Grazin' in the Grass", by Friends of Distinction, was a cover!