I was bored one day on Find-A-Grave back in 2017. End result: I typed in Smith for Last Name and died after 2017; default death year is Exact. Imagine my surprise when I got a few hits. I can’t link to them since some admin or contributor fixed them. It appears to be a case of uploading a spreadsheet with a future death year as you can’t add somebody with a death year older than the current year. In theory, somebody should sort by death year and see if any of the death years are greater than the present year before approving the spreadsheet since it would solve finding those who died 2999 to 20000+. In most cases, it tends to be a case of the person submitting the spreadsheet hit an adjacent number that makes the death date in the future. For example, I believe the contributor hitting 2 instead of 1 created the 2999 death year I saw one time.
In late 2012, I did run across a headstone on BillionGraves where the death date was in the future: October 2017: https://billiongraves.com/grave/Kent-Sidney-Smith/2820973. The photograph was taken almost five (5) years before the death date engraved on the headstone. My first thought was engraving error, but what part of the year was incorrect? It took a while before I found a friend of Kent who said it was his sense of humor and he had no plans to change the date regardless of when he died. Kent was still alive and well as of mid-2013. It didn’t stop somebody from adding him to Find-A-Grave in March 2017 using 2016 as the death date: https://www.findagrave.com/memorial/177162095/kent-sidney-smith. The gravestone photo clearly shows October 2017.