A.K.A the Rule of Credibility: "The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time." - https://en.wikipedia.org/wiki/Ninety%E2%80%93ninety_rule

Followed by Hofstadter's law: "It always takes longer than you expect, even when you take into account Hofstadter's Law." - https://en.wikipedia.org/wiki/Hofstadter%27s_law