" If you've been in the software business for any time at all, you
know that there are certain common problems that plague one
project after another. Missed schedules and creeping requirements
are not things that just happen to you once and then go
away, never to appear again. Rather, they are part of the territory.
We all know that. What's odd is that we don't plan our projects
as if we knew it. Instead, we plan as if our past problems are
locked in the past and will never rear their ugly heads again. Of
course, you know that isn't a reasonable expectation. "

― Tom DeMarco , Waltzing with Bears: Managing Risk on Software Projects