So the silliest thing in my case was as a kid watching the X-Men Saturday morning cartoon.
I actually thought Wolverine could create electricity with his claws. Sound silly? Watch the intro again
https://www.youtube.com/watch?v=sAkL2-vh2Sk
It wasn't until I started really getting hardcore into the X-Men that I realized that was just for show. In all my years I've never seen him do anything with his claws other than slicin' and dicin' (and flipping people off)
What did you get wrong when you first got into the X-Men?