Bad Bond Angle!
There are a few things I insist on in Organic Chemistry. One of the big ones is that I expect students to draw realistic bond angles. I am always reminded that this must be very hard for students to do. Even my best students draw impossible bond angles.
Things look like this:
These present teachable moments. I don't bring the sledgehammer down. Just a calm and stern correction. What they have drawn is wrong, and they can learn from their mistakes with proper guidance.
Eventually, when they should know better, I take points away. I once had a student ask me why I took points away. I told her it was because her answer was wrong. She didn't like that. Apparently she felt her ability to put something onto paper was worthy of credit. I had to convince her otherwise.