I think about this video of French rollerbladers from 1923 at least once every month.
Heelies were never a thing where I'm from, but saw a kid use them while I was visiting a Scottish shopping centre. Didn't even know they existed before seeing them in the wild. A kid rolled few metres before starting to walk normally.
My first idea was that my sensory intake must have been confused somehow, followed by a short second when I started doubting the laws of nature itself.
Thankfully I could see the kid repeat the performance, so could come to sane conclusion.