I once (many years ago) swapped a K7000 (needs an ISO) into a Killer Instinct cabinet, which has a power brick but doesn't have an ISO (because it used a K7400 or U2000 or something which doesn't need an ISO). I didn't realize the Killer Instinct cab didn't have an ISO in it.
The game actually worked fine like that, until I plugged it into a grounded receptacle. Once plugged into a grounded receptacle, it blew fuses, diodes, and burnt up the Killer Instinct's video line on the PCB.
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So my point is, if the guy is testing this by plugging it into a wall outlet with no ground, it may be wired in such a way that he hasn't noticed the need for an ISO yet... but when someone finally grounds it, the machine is going to blow up.