I gave a 2 day estimate to the managers once, and a 10 minute estimate to my peers.
The server side code went smoothly. Couple of minutes, done.

Then I remembered the front end was written under a tight deadline....
Imagine controlling state with jquery, except the state is in html slightly differently to hints in javascript, and there are 7 points of state control, and they have to be triggered in the right order because a few of them depend on everything else, and if you change the wrong line the computer starts pouring smoke everywhere and WHY THE HELL DID I NOT REWRITE IT!