On Sunday, December 6, 2015 at 5:24:04 PM UTC-8, Phil Allison wrote:

The light dimmer was putting an enormous amount of hash on the mains;
somehow it was getting into the microprocessor circuitry.

** Wall dimmers put large voltage spikes on the wiring...

Well shielded, low impedance gear is not affected but anything high impedance and not well shielded picks it up.

There's another possibility: knob/tube wiring puts those spikes on distant
HOT and NEUTRAL wires (not a close-spaced pair). So, there can be significant
magnetic induction, and that can mess up low impedance circuitry. It used
to be seen a lot in CRT television pictures as deflection jitter.

You have to worry about low impedance, too (balanced pair helps).