If for some reason a precinct worker suspects that you're not following proper registration procedures, you're allowed to cast a provisional ballot. The provisional ballot is sealed with your name, address, etc. attached to it. They validate your registration--if you're good to go, they count it, if not, they toss it.
One legitmate use of a provisional ballot is if you request an absentee ballot, but you never received the ballot, so you decide to go vote in person. They see that you request an absentee form, and want to check that you aren't double-voting (absentee + in-person), so they have you cast a provisional ballot and check to make sure you didn't cast an absentee ballot later.
Has ANYONE seen ANy of these idiotic talking heads make an attempt at EXPLAINING what's up with not calling Nevada???? Are they actually so stupid to make us wait hour after hour with NO explanation?