Preparing for a talk on PBT, I added the moving segment time spans property a week or so before the event. At this time, I had used Komposition to edit multiple screencasts. Surely, all significant bugs were caught already. Adding property tests should only confirm the level of quality the application already had. Right?
The classifyMovement function is 30 lines of Haskell code juggling some state, and I managed to mess it up in three separate ways at the same time.