Anecdotal, second-hand story I remembered being told by a former colleague with some significant time working in the Postal service: state or city police pulled over a contracted semi truck carrying mail from one station to the other on suspicion of running un-taxed cigarette shipments from a reservation manufacturer. Because the semi was contracted by the postal service, they had the requisite security: tags, seals and the magical panic button in the cab of the tractor. The driver pushed the button when the state/city police broke the seal and started rummaging through parcels which summoned the inspectors, guns drawn to the scene. Apparently the Postal Inspectors arrested the -police- at that scene on breaking and entering mail facilities.
Some of the details pertaining to why the shipment was actually legal and the police didn't have cause/jurisdiction yadda yadda yadda I don't remember, but still a good story even if the telling of it by my former colleague was more engaging than I can recount.