> [1] I once worked on a system that kept logs of certain types of query so it could display a guess of how long things were going to take and a progress bar to go with it, but this was actually more irritating to the users than no progress display as it would sometime jump from a few % directly to done or sit at 99% for ages (in the end the overly complicated guessing method was replaced by a simple spinner).

In the Tiger era, the OS X start-up progress bar worked this way—it kept track of how long boot-ups would take, and then displayed its best guess based on that.