Say I know where my characters start and where I want them to end -- but don't know the middle. That's where I get down into the thick of things and start plugging along, writing whatever is neat/rationally follows what came before. A lot of the time, I'm wrong about how the sequence of events should go, or else inserted material that isn't necessary -- an evil of the process, I believe, as forward planning would prevent this sort of "Well, it seemed nice at the time, but it really isn't" problem.