This discussion reminded me of the guy who received a $491,000 erroneous refund from IRS -- paid back some of it but was sued for the rest, and won his case (probably now being appealed) because IRS waited too long.  See the blog post here:

https://procedurallytaxing.com/late-filed-erroneous-refund-suit/

What must have happened is that someone else's payment was credited to his account.  Remember, IRS doesn't cash those checks.  They go to a private lockbox, like checks you send for your mortgage or credit card.  I have a client who once worked for a bank operation in Dallas that processed these IRS checks, and she tells me I don't want to know everything that happens in those paperwork sweatshops.  The whole idea is for the government to be able to brag about reducing its size, so they get rid of 5,000 civil servants and replace them with private enterprise that hires 6,000 to do a worse job.