In at least some cases, you have to be halfway into doing the work (or more) before it becomes apparent that there is a big, previously-unknown subtask or requi

In at least some cases, you have to be halfway into doing the work (or more) before it becomes apparent that there is a big, previously-unknown subtask or requirement that invalidates the plan for doing it. In other words, if you really know enough to give a good estimate, you are probably mostly done.