Daniel Kahneman and Tversky found that human judgment is generally over optimistic. People tend to underestimate the costs, completion times, and risks of planned actions, whereas they tend to overestimate the benefits of those same actions. (so called inside view)
A more accurate way is to predict the future by looking at similar past situations and their outcomes. (so called outside view)
Bias correction: Reference class forecasting
As all programmers know, the only thing you can predict about dependencies is that there will be more than you expect Paul Graham
More specific: Planning fallacy