What, exactly do they accept for proof of residency?
And what effort, if any, is made to validate it? I can create and print off a pretty convincing document that I live on the Moon, and pay my water bill to the appropriate lunar authority.
A utility bill in your name, or a bank statement, or tax statement, at the address where you are registering. There is a long list of acceptable docs -- a Kohl's ad doesn't count!
A postcard from “Bed, Bath, and Beyond”, a driver’s license from another municipality, a “letter” from the local office of the Department of Workforce Development (they have someone doing nothing but providing such letters on election day, dutifully typing in the address given to them by the would-be voter), a carbon paper tear-off rental receipt dated that very day, or the word of another voter or the guy driving the van that brought them to the polls.
None of those are currently legal proof of residence, but that does not stop left-leaning poll workers from ignoring the law and accepting them.