Statistics suggest this is more likely than you might care to know. We cannot simply create a schedule and stick to it, and here's why. Every schedule can be viewed as a set of tasks linked together in some order, first to last. Some tasks must happen before others, while others occur simultaneously, and still others must wait until after yet others are completed. Each of these task connections increases the uncertainty within the network of linked tasks.
Every estimate is assumed to be an average value, bounded within a normally-distributed curve.
By the sixth task in this series, the probability of completion within the estimated time begins an endless, asymptotic dance with zero. Yes, you might say, this is obvious. The solution is to pad the estimates, so that we have ever broader boundaries within which to work. This strategy should keep the schedule from falling apart. Okay, let's look at the effect of padding the estimates to maintain the schedule.