Also from The Mythical Man-Month:
Let us consider an example. Suppose a task is estimated at 12 man-months and assigned to three men for four months, and that there are measurable mileposts A, B, C, D, which are scheduled to fall at the end of each month. Now suppose the first milepost is not reached until two months have elapsed. What are the alternatives facing the manager?
3. Reschedule. I like the advice given by P. Fagg, an experienced hardware engineer, "Take no small slips." That is, allow enough time in the new schedule to ensure that the work can be carefully and thoroughly done, and that rescheduling will not have to be done again.
If you need to reschedule, take responsibility - and make sure you only need to reschedule once.
This book is good.