A problem I regularly come across as a field service technician.

Cable shielding disconnected. Customer complains that the machine is going into fault, after a reset the fault clears... Often the importance of shielding is not understood. It isn't just a formality, it protects signal cables from electrical noise. Disconnect it or ground it badly, and you get intermittent faults that look completely unrelated, comms errors, false triggers, sensors reading badly. You can actually see it on a scope, noise on signal lines that should be clean. If you're chasing a "random" fault on a machine, check the shields before you check anything else. Fast and easy first check!