If you let it come to that than you're doing something wrong. It either means you didn't estimate well enough or you didn't inquire about the project time-line. E.g. If you estimate a job to take 40 hours but the deadline is in 2 days then you have a problem. Getting paid for the 40 hours is not going to make you happy if you have to work 2 days around the clock. In other words: Estimating is one thing, but it also has to fit into an achievable time-line.