Hofstadter's Law on Wikipedia
It always takes longer than you expect, even when you take into account Hofstadter's Law.
You might hear this law referred to when looking at estimates for how long something will take. It seems a truism in software development that we tend to not be very good at accurately estimating how long something will take to deliver.
This is from the book 'Gödel, Escher, Bach: An Eternal Golden Braid'.