Lights started flickering occasionally, and them more frequently. I was trying to pinpoint the problem, found fluctuating voltages off my roommate/landlord's main breaker.
As I was standing at the panel, the lights began flickering. There was a squirrel running on the line, and the lights were flickering in rhythm with the squirrel.
The ******* bushy tailed rats had chewed through the neutral (which was a wound aluminum shield around the hots). PoCo had little jumpers that he could attach from the ground, and tighten the lugs with a long fiberglass pole. Did it all from the grounds.
It helped me better conceptualize the issues an improperly wired multi-circuit pose. (I post alot about those.)