Teachers skipping steps when showing a math problem because, "it's common sense."
I am convinced that math people can neither teach math to non-math people, nor understand that a non-math person could possibly exist upon the earth.
In school or college, did you ever experience a teacher skipping over multiple steps in a problem on the board, and when you asked how these numbers came out of nowhere, the teacher responded that they skipped steps because it was basic common sense?
I recall this happening several times. It was so frustrating. I am doing everything to keep my head above water but the teacher is smugly beholding truths to be self evident. It really just reinforces the desire to give up. The teacher is dismissive toward you and is not willing to show their work, but they expect a kid or young adult to understand at the same level as someone with a MS/PhD in mathematics.
I have been told multiple times that "no one is bad at math, you just arent applying yourself." Ok but someone who is bad at sports can be uncoordinated or considered a non-athletic and that is fine. We don't berate students if they dont have a 30 inch vertical in basketball and tell them that everyone can dunk if they just paid attention.