I used to think Dream On was by Led Zepplin instead of Aerosmith. And that song Forsaken, I'm still not sure if it's by disturbed or korn. I don't really care enough to check.
I used to wonder why "Soley, Soley" was never on any ABBA Greatest Hits albums until I found out that it was a song performed (in English) by Middle of the Road. (The French version, "Soleil Soleil", is more famous, originally recorded by Nana Mouskouri and then a couple of decades later by Lara Fabian.)
When I was a kid, I thought "Happy Together", by the Turtles. was a Beatles song. I think I only knew about four white bands with male vocalists from the 1960s, the Beatles, the Rolling Stones, the Who, and the Beach Boys, so white-sounding songs from the 1960s obviously had to be from one of those four bands or maybe Elvis Presley.
I thought "Somebody's Watching Me" was strictly a Michael Jackson song. Only when I was in my 20's thanks to the power of the internet did I learn that it was a Rockwell song that only featured MJ doing the chorus
I had no idea Pearl Jam's "Do the Evolution" was Pearl Jam for quite a while when I was a young teen, since to me it never sounded like any of their other stuff. Then I realized that I actually had no idea whose it was. Didn't sound like it belonged to any of the big name "90s grunge" bands. Curiosity lead me to the truth and I felt dumb.