I had a weird electrical problem and found the fix, but I'd love to understand the mechanism by which this happened. Disclaimer: I'm not an electrician, buyer beware, etc.
The symptom I had was 3 separate Siemens AFCI breakers tripping, seeming randomly. I spent a lot of time puzzling over this, and eventually noticed that something weird was happening with the 240v heater (smart line voltage thermostat) that was also fed from the same sub-panel. For context: all these breakers are in a sub-panel fed from the main panel. The sub-panel and main panel are standard two leg 240v where you get 120v to ground from each, and 240v between the legs.
Checking the voltage at the heater, I observed 11 volts across the hot legs, where I would expect 240v. (but 120v from each leg to ground!) I chased the 11v up the chain, and discovered that the breaker that feeds the sub-panel was somehow "half" on, such that (I think) it was only electrifying one leg for the entire sub-panel. That in turn caused a 240v wall heater that was fed from the sub-panel to go haywire -- it would start up to deliver heat, die, and repeat.
Other 120v circuits in the sub-panel were not affected. Amusingly, there's also a dryer fed from the sub-panel and it spun happily but didn't dry clothes. (I didn't notice the wet clothes until after all this troubleshooting) Apparently the motor runs off a 120v leg but the heating coil is 240v. (not 11v!)
Anyway, I pushed the sub-panel feeder breaker to "fully on", and everything was fixed. And I lol'd. But it would be interesting to better understand the mechanism by which this all happened. I have two theories that I put in answers.