What's a piece of 'common knowledge' in your field that you're pretty sure is completely wrong?

In my field (small B2B software), the common knowledge is "you need a sales team once you pass a certain revenue threshold." Everyone says it like it's physics. But the two most profitable small software companies I know personally have zero salespeople the founders just answer support tickets and the customers upsell themselves. Meanwhile the companies I've watched stall out all hired salespeople too early and then had to feed them deals that weren't ready.

I'm not saying sales teams are useless. I'm saying the threshold gets repeated as a law when it's actually a guess, and a lot of people make expensive hiring decisions based on a guess.

So what's the "everybody knows" thing in your field that you suspect is just inherited folklore? Bonus points if you've actually tested it and been wrong either way I'm more interested in the people who believed the common knowledge, tested it, and found out it was right after all.