Industrial but guess it counts.

Giant motor is supposed to kick on, run for a moment in reverse, wind down, and then go forward. What is happening instead is it kicks on then the whole system goes into stopped state. Two days on the phone and I can't figure it out, pouring over the code, trying everything.

Suddenly the guy in the field coughs and says "sorry it's really dusty here".

It clicks in my head. I tell him to manually push down on the contactor. He says he feels resistance I tell him that's good and push harder. It give in and I tell him to start again. Works perfectly.

The dust had combined with the internal oil of the contactor making a sludge. The contactor has two coils, a high torque high current one for starting and a low torque low current one to hold. Not much different than a starter in a car. The sludge has stopped the second coil from engaging keeping it locked in high current. Since it was DC the coil kept drawing more and more amps until the power supply couldn't keep the voltage high enough. Which made the PLC halt. When the PLC halted it erased all the temporary bits including the one that said it was running. The PLC stopped telling the contactor to engage and the power went back to normal.

The sequence was maybe a tenth of a second.