This is a problem that happens to me a lot. I pride myself in being a pretty flexible artist, having good comunication with the client and overall getting a good picture with as much accuracy as possible. However this has lent itself to several clients going a little overboard with changes to the point of being a headache, especially when they want something changed "but they don't quite know why/what" or "this line looks out of place, please change it"
I know there are several ways of dealing with this problem, such as charging after a certain amount of changes, but it leaves me a bit sour in the mouth since it can suggest to potential clients that you might purposely mess up in order to charge them extra. Other options include charging these nit-picky individuals extra just for being them or refusing business from them outright.
My TOS is due an update and I would love to address this issue. If you could share with me your methods or suggestions it'd be much appreciated!
EDIT: Thank you very much everyone! I'm really glad my concerns were unfounded. I think I will go as many of you suggested with a numbered free changes before charging, especially on sketch phase. Again thank you very much!