There was something about the map that Wormtail and Moony didn't know about. It was that in their Fifth year James and Sirus put sometime turner dust on the map.
If someone committed their 500th prank with the map and if the prank involved Severus Snape then James and Sirus exactly as they were when they put the charmed dust on would be brought to an alternate universe with the other pranksters.
The alternate Universe was called Marauder World. There were people from both time periods there. Both group would get a copy of the map and an invisibility cloak. They would start in a prank war. Dumbledore would judge. Then they would go back to the exact moment when they left.
*Only James Sirus and the other people left their own world. The other people were Alternate-Universe-People.