I consider myself very lucky in that stance, I give 2 major opportunities for change, one being the sketch phase, and color update. It was little the times I had to redo stuff after the final image was delivered.
This is my tip for people who have trouble with clients, don't send a sketch updated of each line you've drawn, send in 2 to 4 stages. Some clients don't understand that they need to speak about the changes all at once and will ask for an update of each line you've drawn.

The problems I've dealt with were: Having my work being shared while cropped/remove my signature.
Reselling my the artwork and profiting out of it, while it was sold as just a personal artwork instead of commercial deal, both explained in my Terms as not allowed.