You might have everything planned and scheduled, with smaller tasks delegated so the whole project will be done right on time. That's great -- until a crucial member of your project team misses a week with appendicitis, someone's hard drive crashes and loses vital files, or the client calls and amends the project criteria. Unexpected events can derail even the most solid plan.
Of course, you can't plan for every possibility, so the best defense is a deadline cushion. If it's a two-week deadline, plan to have it done two days early. A two-month project could use a five-day cushion. The more uncertainty built into your project, the larger your cushion. Setting this kind of soft deadline will give you a crucial grace period to deal with unforeseen problems. You might even have time to recover from outright catastrophes.
Imagine how good it will make you look when you can explain how you recovered from an office flood that ruined some files instead of using it as an excuse.