Back around 1980, and fresh out of university, my first job was for a company making control systems for printing equipment (Crosfields). A customer asked us to help with a static problem and, as part of my training, my boss took me with him to look into it.
The customer was printing the wrappers for fish fingers, using a toluene-based ink onto cellophane. The cellophane passed from one roll, through the gravure printer, onto another roll (each roll a few thousand feet long). There had been a history of explosions, thought to be caused by static. These were tolerated until the local union boss took a look round, peered into the machine, and lost his eyebrows when it blew up in his face.
He immediately called everyone out on strike, which was when we were called in. When we arrived, my boss handed me an ancient field-strength measuring machine and told me to take a look round, while he went to speak with the managers.The printer had been started up for our benefit, so I waved the meter near the take-up roll. The dial immediately swing up to around 300kV/foot (almost 1MV/m in modern units).
When my boss came back, I reported that to him, and showed him where I had taken the measurement. He didn't believe me and pointed to the machine in disbelief. "Don't be ridiculous. It can't be that high. You must have made a mistake. Look at that th....". At that moment, a thunderbolt shot from the roll to his outstretched hand, throwing him across the room.
He slowly staggered back to his feet "Yes. OK. I get the point. We do have a static problem".
In retrospect, it should have been obvious. The setup was effectively a Van der Graff generator. Static was being generated by the cellophane flexing as it ran through the printer and it was steadily accumulating on the periphery of the take-up roll, which was acting as the dome of the Van der Graff generator. The big trough of toluene-based ink was just waiting for a spark to set it off.
I think the problem was eventually fixed with some carefully-placed electrostatic "tinsel" placed next to the rolls (Icon depicting what was happening there several times a week)