Beware of scope creep. If it's a bug, fix it for free. If it's a new feature or changed from the original signed off scope, charge for it.

Related: Remember that it’s the customer’s job to set the scope with sufficient specificity. You can guide them and help them be aware of things, but ultimately what is agreed upon at the end is the guide, to the letter. Otherwise you get stuff like, “Well, when we said it’d have a menu I *thought* it was going to be a multi-level, dropdown menu that changed preview images with every link you hover over, not a bland set of links on a bar in a single row!”

Start giving in to clarifications like that and you’ll never hear the end of it.