Eliezer Yudkowsky. Harry Potter and the methods of rationality (№ 415263)

Muggle scientists have found that people are always overly optimistic: they say that some process will take two days, but in fact it takes ten, or they say - two months, and it takes more than thirty-five years. Or, for example, a survey of students was conducted, by what date they are 50%, 75% and 99% sure that they will complete their homework. And only 13%, 19% and 45% of them complete it by the specified time. Scientists have discovered the cause. The subjects were asked to describe the ideal and typical scenarios. And the resulting descriptions were almost identical. If you ask a person to plan something for the future, then he usually, imagining the most likely course of events, forgets about the possibility of mistakes or surprises. Most of the subjects did not finish the work by the deadline of which they were 99% sure, so that the actual results were worse than even the worst-case scenario. This phenomenon is called a "planning error," and the best way to avoid it is to consider how long it took to do any work in the past. That is, to look at the process from the outside. If you're taking on something for the first time and there's a possibility of failure, you have to be very, very, very pessimistic. So pessimistic that the results accurately exceed expectations. I, for one, work hard to present a grim picture of one of my classmates being bitten by a monster, but it could actually happen that the surviving Death Eaters attack the school to grab me. It's good that...
"Enough is enough," McGonagall interrupted.
Harry fell silent. He was just going to add that they at least knew that the Dark Lord wouldn't attack because he was dead.