The planning fallacy

The planning fallacy is when people forget to take delays and obstacles into account while planning. This happens because it is impossible to predict which obstacles will come up while making the plan, so they don't get included at all. But it is very likely that some obstacles and delays will occur even if it can't be foreseen which ones during planning. Therefore one should ideally plan for extra time to handle unknown problems.

When people are asked to make best case, average case, and worst case estimations of how long it will take to complete a task, their best case and average case are typically close to each other. In reality, the task almost always takes longer to complete than the worst case estimate.

To beat the planning fallacy

In order to beat the planning fallacy, it is necessary to take unknown obstacles and delays into account when planning. A pragmatic way to do this is to take the outside view: Pretend to be someone else and simply ask "how long did it take previously to complete the same or similar projects."