Some years ago, my team created a web-based portal for a local ad agency. Long-running actions that triggered ajax calls would display this hideous, too large, black spinning wheel. Probably there was a bug that caused the spinning wheel to sometimes remain on the screen until you refreshed.
At one point a user contacted us and asked how long the "black sun" as she called it was supposed to be there? We shrugged our shoulders and thought, "maybe a few seconds?". She said she had been working in the app, navigating and clicking right under the spinning wheel, for two whole days before contacting us!