A large distribution center had a heavy-duty electric counterbalance sit-down forklift that started acting possessed. It would drive perfectly fine in reverse, but as soon as you put it in forward and pressed the accelerator, it would lurch violently, the "motor fault" light would flash on the dash, and the truck would completely shut down. If you turned the key off and back on, it would do the exact same thing.
Because the failure only happened in the forward direction, the maintenance team wasted a full day checking the forward directional contactor and the accelerator potentiometer. Both tested perfectly fine. The issue actually lay deeper inside the AC (Alternating Current) drive motor itself.
Unlike older DC motors that use physical brushes to transfer electricity, AC motors rely on a motor controller to rapidly switch the polarity of the power going to the motor windings. To do this precisely, the controller needs to know the exact physical position of the motor's rotor at all times. This job falls to a tiny circuit board called a Hall Effect sensor, which sits just inside the motor housing, reading the magnetic field of the rotor. Due to the extreme vibration of daily forklift operations over several years, the solder joints on this small sensor board had developed microscopic cracks. The sensor was failing to send a position signal to the controller at the exact moment the forward current was applied. The controller, realizing it didn't know where the rotor was, panicked and shut the truck down to prevent the motor from self-destructing. Reverse worked fine simply because the electrical phasing sequence hit a slightly different, less-damaged part of the cracked solder joint. Replacing the tiny Hall sensor board required pulling the motor, but the part itself only cost forty dollars. It's a classic case of modern forklift diagnostics: a forty-dollar electrical component masquerading as a catastrophic transmission failure.
