This isn't the first time this has happened by any means, but it never really gets old.
MSNBC reports that a 29-year-old man driving a white pickup truck made an unauthorized traffic stop earlier last week in Upper Marlboro, Maryland. He told the driver he was a military police officer from Louisiana and that he had stopped the driver for speeding. The driver responded that the alleged MP was "out of his jurisdiction" and also that having emergency lights on his truck was against the law in Maryland.
The driver knew this partly because unlike the guy who stopped him, he actually was a police officer. (He was off-duty and driving his own car, although that makes the stop only slightly less baffling.)
The report then says simply that "the suspect left the area," probably rather quickly, and that the officer called for backup and followed the suspect until Prince George's County police arrived. They charged the suspect with impersonating a police officer, and I am charging him with doing it poorly.
Previous impersonations mentioned here have included this failed attempt to impersonate an air marshal on a plane occupied by real ones, and this remarkably successful attempt to impersonate a Chicago police officer, which fooled real ones for several hours although the culprit was only 14 years old. (He was big for his age, they said.)