Every election, particularly a highly contentious one like last week, we have voters show up which are not registered in the precinct where I am charged with being the election judge.
They swear up and down that they have voted here for years. Once in awhile they are out of alphabetical order in the sign-in book. Most of the time, once we show them our precinct boundaries, they figure out they aren't supposed to be here or we call the county to find out for them where they are supposed to be and hand them a registration form and an admonition to get them corrected before next election.
It is not a free for all where you just waltz in and go to a voting machine no questions asked.