@deanb In addition to all the really shady stuff like "losing" registrations or whatever (which does happen, but I think people blow it out of proportion) recently states have started passing laws where if you haven't voted in the last X number of elections then they send you a notice to check that you're still living there, and if you don't respond they unregister you. It's not a problem for people who vote regularly, just for people who registered years ago but haven't actually been voting, then decide they want to, only to find out they've been unregistered.
I actually had a weird thing happen to me in the primary this year, where they thought I had moved and had changed my registration address, even though I hadn't. Turns out the post office sent a change of address for a different person and they got it mixed up with me and changed my voting address instead. I truly believe that was accidental though, because they were really helpful about it and even made it a point to call me later after they'd figured out what happened. I had my wife check to make sure she was still registered properly, and she was.