When I first heard The Brotherhood Of Man's "United We Stand" I thought The Partridge Family recorded the song-even though the PF TV show debuted a few months after UWS came out, I only heard it after its initial release. Also, I thought Edgar Winter's "Free Ride" was done by Janis Joplin [three years after her death]. Any such examples?

I thought "Laugh Laugh" was done by The Beatles...instead it was done by The Beau Brummels who were a West Coast Band!

When I first heard "Suspicion" I thought it was Elvis. It's actually Terry Stafford. Elvis did do his own version though.
http://www.youtube.com/watch?v=TUe7dMi2uLA

"When You're Only Lonely" - thought it was the Eagles with Glenn Frey singing lead but it's JD Souther, who did write songs for the band.
http://www.youtube.com/watch?v=PVjKZBriACo&feature=related

And all of Badfinger's songs I thought were done by The Beatles.

Speaking of which, I thought these guys were...

Somebody To Love

I thought it was performed by Janice Joplin, but it was Jefferson Airplane and a funny rendition by Jim Carrey in The Cable Guy of which is on the soundtrack which isn't half bad.