The customer wants new work done on the project - work that isn't currently part of the scope, of course. Or perhaps the project hasn't started yet and you're still putting time and energy into good solid work estimations that will turn into timeframes of effort in your master schedule. The possibilities are endless.
When situations like this arise, as they often do, what's the key to staying on schedule and budget AND ending in the black - profitably speaking, that is?...