Everything costs twice as much, and takes twice as long as you expect.
The “Iron Law of Megaprojects” according Bent Flybjerg is “over budget, over time, under benefits”1.
Projects are driven by psychology and power1.
Projects often have a “fat-tailed” distribution (as opposed to normal distributions) where the tails have extreme outcomes. That is, costs aren’t just going over, but theres a non-trivial chance that they go disastrously wrong1.
Projects of any meaning size tend to exist in an environment of complexity and interdependencies. Therefore, disruptions tend to ripple through the system1.
1. Flyvbjerg, B. & Gardner, D. How Big Things Get Done: The Surprising Factors That Determine the Fate of Every Project, from Home Renovations to Space Exploration and Everything In Between. (Currency, New York, 2023).