Stupid questions aren't a myth. They do exist and they aren't even rare. I'm pretty sure it's more likely you get asked a stupid question than you get hit by a car.
So what qualifies as a stupid question? A question:
- to which you can deduce the answer yourself by applying logic.
- outside of the area of expertise of the person you are asking.
- for a well-known fact.
- so broad you already know it will have a sophisticated answer.
- that you've already asked, but forgot the answer to.