Michael Wolfe's answer to the Engineering Management question, "Why are software development task estimations regularly off by a factor of 2-3?" is entertaining, and almost certainly more
wisely widely (well, the typo don't lie) applicable.
Not that I'd know anything about schedule slippage, of course.
P.S. He is CEO of Pipewise, and you should have a look at the beta tester sign-up form on their home page. Ah, Mad Libs.