Dudes claiming they are "old stock" or "heritage" Americans. They'll even call other white people who's ancestors weren't here before the Civil War "ellis islanders" and invaders.
As someone who's ancestry has partially been here since the 1690s.. it's a crock of shit and silly as fuck.
There's lots of places that have surviving records of births, marriages and deaths of commoners stretching that far back and often further than that. For instance most Icelanders have genealogical records dating back to the settlement of Iceland in the early middle age.
Also, the longer back you go, the sheer number of ancestors means that the likelihood increases of one of them being a nobleman whose genealogy would be much better documented than that of a peasant.