Mythconception - "mith-con-sep-shun" - noun - A
misconception based on a myth. Although it sounds like you've got a
lisp, this is often a very apropos term for someone's
misinformed opinion.
Example:
"A common mythconception is that you shouldn't go swimming for at least half an hour after you eat"
"No, actually, that's just a mythconception.."