Every culture has its own set of persistent myths that are passed down the generations. I'm not talking about gods and monsters, though, I'm talking about myths about language. Myths are like optical illusions -- once you know what you're looking at, it's obvious to you, and you can't imagine how you missed it.
As teachers, we should be wary about passing on linguistic folklore uncritically. We should look at everything we've been told about our languages in detail to see whether it holds up to scrutiny, and if it doesn't, we shouldn't teach it.
I'd like to give you an example from the teaching of Scottish Gaelic.
If a course teaches noun cases explicitly, it will state that the "second noun" is always in the genitive. It will then go on to add the acception that if another noun follows it, it isn't in the genitive, so that in a long and complex noun-phrase, only the final noun is in the genitive.
So it is self-evident that "the second noun takes the genitive" is an incorrect rule. The actual rule is "the last noun in a noun phrase takes the genitive". Once you see this, it is obvious, but the "second noun" rule is now so all-pervasive that I'm currently hearing it in grammar classes aimed at fluent speakers in their second year of university study.
The myth is being passed on to a new generation.