I suspect it might be theoretically possible for ancient computer games, of the SpaceWars or Adventure eras, to have lapsed into the public domain by omitting copyright notices or lapsing registrations.
This was possible before a change in copyright law in the US in 1988 when copyright became more or less automatic. The cutoff date was January 1st 1978, which does exclude most modern video games, and people didn't much care about copyright in video games, or software generally, prior to that date.
There are a few films made relatively recently - Night of the Living Dead being a prominent example - which are public domain through oversights in the handling of the film's copyright. Whether there are actually any video games that went public domain by this route is left as an exercise for the reader...
They finally "solved" the copyright problem with Night of the Living Dead by doing a new restoration, with a fresh digital transfer and some fixed audio. So they can claim copyright on that, just not the original material. I suspect it'll be released on blu-ray soon enough.
This was possible before a change in copyright law in the US in 1988 when copyright became more or less automatic. The cutoff date was January 1st 1978, which does exclude most modern video games, and people didn't much care about copyright in video games, or software generally, prior to that date.
There are a few films made relatively recently - Night of the Living Dead being a prominent example - which are public domain through oversights in the handling of the film's copyright. Whether there are actually any video games that went public domain by this route is left as an exercise for the reader...