Sometimes we purposefully make computers do nondeterministic things on purpose. Hardware random number generators, for instance.

But not counting those situations, speedrunners often call some of the "other factors that just absolutely cannot be accounted for" kind of stuff that rethnor is talking about "RNG".

But even aside from that, there are some even less deterministic cases. Also from speedrunning, there's the famous SM64 upwarp that my have been, but probably wasn't a cosmic ray, but very likely was some kind of hardware failure that wouldn't be accounted for by only considering how the N64 is supposed to work (given processor architecture and such).

But an example I find even more interesting. There was a Defcon presentation where someone registered domain names that were one bit different from very frequently used domain names, kindof like typosquatting, but these were for domain names that no human was typing. These were domain names that computer programs were hitting, and the domain names weren't derived from user input. But his one-bit-off domain names got a significant amount of traffic. Just from random memory corruptions and such due to some variety of weird hardware failures.

I'd be skeptical of anyone I know claiming that they'd actually personally encountered something they knew to be truly non-deterministic in hardware that wasn't in some way temporarily or permanently faulty or damaged in some way, but I think it is a thing that can happen in computers. And some of that nondeterminism comes from sources that actually do act at quantum scales were true nondeterminism isn't unlikely to have sway.