I think I linked to the author of a question who no longer is a registered user†, and is now displayed as "user6163". That might explain some re-use of network IDs to keep them short. Still then, if doable: the old GUID URL should probably yield a 404 instead?
† Given a Meta post about lost reputation for the old user "Nir", maybe some abuse was taken care of, and the current user is some merged account. But then I'd expect some activity in Mathematics again?