They managed to sneak an image of a pig into a state police crest that is emblazoned on police cars.
About 30 cruisers sported the design for the last year.
The official crest depicts a spotted cow against a background of snowy mountains, but the inmates' version featured one of the cow's spots shaped like a pig in an apparent reference to the slur word for police.
The decals now must be removed and reprinted.
The prank does come at a cost to taxpayers. New decals will run about $800.
