I used to think Barkley from Sesame Street was a real dog
Before I knew how the Big Bird puppet worked, because he's so tall it didn't make sense to me that an actual person would be inside so I assumed Big Bird must have been a robot.
I originally thought Olivia appeared in the first season of SST.
I used to think that the first few seasons of SST were filmed in black and white.
I used to think Brian Henson played Kermit after his father's death.
I've been a Muppet fan since I was a baby and found out about many of the performers when I was five, but I didn't know that Eric Jacobson had taken over Frank Oz's characters until I was about nine.
I originally thought that Beautiful Day Monster was an early version of Sam the Eagle, due to their similarities in appearance.