What's a market belief that sounds smart but often leads people astray?

Some ideas sound intelligent.
People repeat them constantly.

But following them blindly can be dangerous.

What's one investing or market belief that gets repeated so often that people stop questioning it?

Examples:
"Time in the market beats timing the market"
"This asset is a safe haven"
"The market is efficient"
"Diversification solves everything"

Which belief deserves more scrutiny?