Well, after the fact it's realized the person whose name is on the ballot had been dead for months, and you didn't catch who cast the vote because you don't know who they are.

Or a million other scenarios. Really all it takes is stopping to think about it for more than a minute.

I mean if they registered with some identifiable piece of info (SSN for example) then you would see two votes with the same SSN if they were pretending to be someone else and if they dont have an SSN then they arent a citizen.