We have a piece of specialized production equipment that used to on occasion refuse to start up without a bit of percussive maintenance (a.k.a. "Fonzie treatment"... ask your parents, kids). We discovered the fix accidentally. The first time the unit refused to start, we anticipated a breaker had tripped. To verify, we needed to open the access panel, which required the use of a coffin key style tool. Said tool was attached to the machine via a loop of 30AWG wire. While untying the key, I fumbled the key which (still attached to the wire) impacted the side of the machine, causing the lights to blink on.
We spent months trying to find the root cause. The failure was very intermittent: the machine would work for days or weeks with no problem, then would fail randomly. We tried replacing relays, tightening screw terminals, etc. No luck. In all cases, once the machine was "Fonzied" it would work all day long.
Eventually we discovered something: our plant wiring had essentially all our solder stations, heat guns, work table lighting, and this piece of equipment on one 20A circuit. If the right combination of equipment was on, the 120VAC was closer to 90 VAC at the outlets. The equipment in question had a step down transformer that generated 24VAC for the control circuits. Said control circuits included a start button that would pull in a 24VAC relay that allowed 120VAC to the motors and other main functions of the machine. That relay would hold just fine at about 18VAC (what you get if the input is about 90VAC), but wouldn't pull in (unless a physical jolt got the armature moving).
Bummer, because I had just started to perfect my "Ehhhhhh" to the point that I was going to shop for a leather jacket.