Henrik spent too many hours, staring at the bug, trying to understand why the 3rd party service they were interacting with wasn't behaving the way he expected. Henrik would send updates, and then try and read back the results, and the changes didn't happen. Except sometimes they did. Reads would be inconsistent. It'd work fine for weeks, and then suddenly things would go off the rails, showing values that no one from Henrik's company had put in the database.
The vendor said, "This is a problem on your side, clearly." Henrik disagreed.