Machine had been down for several months.

Replaced a comms cable to get everything back online, then the fun started... Running production, nothing was reliable, signals jumping all over the place. Time to make a thorough inspection. That's when you find the small stuff. Loose mechanical brackets, shielding disconnected, sub D plugs not secure, a missing screw on a light emitter for an optic sensor. On their own these issues aren't a show stopper, but added together you get a very unreliable machine doing weird things. When it's a breakdown the priority is getting back into production, not checking every screw and bracket. But spending the extra time to do a proper job pays off in the future.