Here are some "facts" I've heard during my musical career that I later discovered to be false. I'm sure there are many more; these are just the ones that I've remembered during the last 5 minutes or so.
"You can't ever look at your hands while playing piano."
"This chord voicing is hipper than that one."
"Louis Armstrong was wrong to use celeste on his recording of Basin St. Blues."
"Nice guys finish last."
"Elvis is pop, not country."
"West Side Story is corny."
"Miles Davis composed the tune Blue In Green."
"Always keep a steady tempo when practicing sightreading."
"This is how to play Chopin."
"Improvisation cannot be taught."
Do you have your own list? If so, we'd love to read it in the comments section below!