I'm not a teacher but from my days back in university, I remember the most common mistake people used to make was either the addition of extra lines or forgetting them.
Because usually a line indicates the connection between two carbon atoms, some people mistakenly assume that the atom is in the middle of the line instead.
Sometimes people makes the mistake of getting extra bonds or even extra carbon atoms that does not required.
That's the only thing I can remember that took marks off our grades back then. I hope the others can add extra common mistakes that I'm not aware of.