Chapter #57. Never Show an Animated, Looping Progress Bar
Some of the most awful (and hilarious) UI disasters that I've come across have this particular travesty in common. An animated (often a GIF) progress bar which, once it has meandered its way to the end, restarts back at zero and does it all over again is common. Think of it as a "linear spinner". If you really dislike your users, then this is an excellent way to "troll" them.
Animated progress bars could actually be a symptom of developers testing things locally on their computers, rather than on the wider internet. Everything loads so quickly during local testing that a developer may never see a progress bar. This is yet another vote for testing in the real world with real users (refer to #101, Test with Real Users).