I work in the IT field... it happens all the time...

the ONLY project that I've been involved with or have seen/waited for that actually finished ON TIME was the Y2K project....

It is RARE for ANY project to finish on time as originally expected. It's called life. No matter how well you plan, life will always throw you a curve you weren't expecting. REAL managers do not judge you on whether or not your miss a self-appointed deadline, but instead will judge you based on your explanation on why you missed the deadline, and will really judge you on how well you handle that curve ball that pushed you over the deadline.