That does sound awful. Especially for the students not confident in their answer, finding it's different than the back, spending time trying to get that same answer. I've had times like that where, once I got the right answer, I had convinced myself of the right way to do things and similar problems aren't as hard anymore. In the case of wrong answers, I might convince myself of something that is false.

This is why I think grading on work is more important than the right answer, though of course the right answer and right work should be taken into account. If problems are such that little to no work needs to be shown, they aren't worth assignment as homework.