Never gave it much thought, yet I've moved a bunch of times, and to different states, and registered to vote each time, though don't recall ever "unregistering" anywhere. Not sure how that works, I assume there's something we're supposed to do that I've never heard before. But, I guess there's no harm done so long as a person only votes once.
Don't know how other states do it -- in Missouri, I receive a voter registration card in the mail (once a year, I think), which verifies I'm registered, where and how I'm registered, and where to vote -- that registration corresponds to a "log book" maintained at the voting facility, where I have to show who I am and sign off before receiving a ballot. Of course, if the clerk doesn't know me and doesn't ask for identification, I guess I could say I was anyone, so long as I gave them a name contained in the log and I signed the pre-printed name.
In some localities, counties and/or states, people are allowed to get drivers licenses without any proof of citizenship, or in spite of non-citizenship, are allowed to register to vote during the same process, and are then allowed to vote in some (or perhaps all) elections. This is possibly totally legal in some places, at least for local elections, but not federal elections. Not sure how large of a problem this really has been, yet it has been a documented problem in some close elections, and some have gone to prison over it.
Of course, there's always the talk of those voter registrations in the names of deceased people, who somehow have continued voting.
And, I don't recall where it was, not even which state, but I remember reading somewhere that there was a county that actually recorded more votes than the total number of registered voters. Not sure how that came about, but it couldn't have been legal.
