Apparently the McCain camp has sent out over a million misleading absentee ballot applications to likely Democratic voters in the purple states.
If these applications are sent as registered mail and are not picked up out of the mailbox within a certain amount of time, the recipients can be challenged on election day.
Or, if they are returned without checking an unneeded "I am a registered voter" check box, then these recipients can be challenged at the polling place.
This is vote caging and it is illegal.