In order for the fake ID to work there must be certain conditions to prevent it being found out after the fact.
The fake ID has to be a spoof of someone already registered to vote and the real person must not show up to vote before or after the person with the fake ID votes.
The fake ID must be for a fake voter registration. That voter registration must have the ability to be mailed a voter registration certificate to a valid address that won't be returned or questioned.
In the first case, there are instances of people showing up at the polls to vote and be told they have already voted. These cases are few and are often mistakes by poll workers in registering who voted.
Voter registration for invalid addresses also occur. The voter could have moved before the registration certificate was sent out. Voter information could have been entered wrong by government workers. The voter may have made an error in writing down the address. There have been a few cases of voters not understanding the registration process such as writing the street address of where they have their PO Box as where they live. There have also been cases of fraud using this process, one instance where the owner of a strip club had his employees file to vote with the address of the club so he could win a local election.