Depending on your state they can also purge the voter rolls and just unregister whomever they want at their whim. Ive had family members and friends get purged and, somehow, the notice that they were purged just magically never showed up in the mail, so they would go to vote and be sent away at the polls. As a result I have to manually recheck my registration before every single election. Each state is totally and completely different in this regard, you cant take any assumption from your home state and extrapolate it to others.