By the third crash, he had noticed something. The crash occurred when the attendants were changing the tapes on an unrelated drive. And furthermore, he realized that the crash occurred as soon as one of the attendants walked across a certain tile on the floor.
See also More Magic.
There was a time I had to chase down bugs like this. E.g. the system where we had to use an extender card in order to physically plug in the emulator, but installing the extender sufficiently altered the system’s electrical properties to change its timing and behavior. By and large I am very happy to have left that world behind, but working on cloud-based web apps does reduce a developer’s opportunity to occasionally demonstrate apparently godlike powers of deduction.