We have no way of knowing what happened with Kean's meter but I would not rule out high current / high voltage. If I personally had damaged the 121GW like this, I would own it. I may even try to replicate it for a video and then try to improve the design.
Yeah, I'm happy to "own it".
As I've mentioned in the YT comments and 121GW forum post, it looks like I screwed up and used the 121GW when I shouldn't have - measuring the HV output of an ultrasonic transducer driver. Roughly 1200V at 25-28kHz, at potentially 100W!
I don't remember doing this, but it really seems like I must have.
And I very likely didn't use this meter again till I went to measure that flakey DC/DC converter a week or so later and saw 166V, then OFL, instead of 5V. It measured more like 3.5V on an EEVblog/Brymen 235 meter, and that was fixed with some extra input capacitance.
I tried to reproduce the damaging arc on a test PCB - an SMD breakout with approx the same 1mm trace gap. I was set up to take video and everything, but it just wouldn't arc over by itself, and I wasn't game to manually start the arc like Joe did. Most likely would have killed my driver board.
And I'm also happy to say that the meter survived after cleanup of the PCB. Even the protection diodes that are in the path seem to measure fine (forward voltage and leakage).
Thanks again Joe for the excellent series of testing videos you've been producing.