I boarded the wrong flight once, traveling from PHL to ORD via CVG about 5 years ago, they were boarding the ORD segment and another flight through one gate at CVG, when you got to the bottom of the stairs the covered walkway split in two directions, my luck I picked left instead of right. I get on the plane, take my seat, and before they close the door they announce the flight is going to LEX. Yikes! I go up to the front of the plane, explain what happened, the pilot called operations and had them bring the ORD flight (which had just pushed back), to return to the gate, he then went into the cargo hold, found my carry-on bag for me, and walked me over to the correct flight. Amazing customer service! I got the pilots name and wrote a very nice letter to DL thanking him. It was the most embarrassing flight in my 30 years of travel; needless to say there were not too many friendly faces on my fellow passengers on either aircraft. I did notice that a few weeks later they started checking boarding passes at certain airports when you got on the RJs, I guess they enacted the BergerPHL rule.