I was scheduled to fly IAD-JFK this morning on Freedom Airlines (aka Mesa for Delta) and experienced a delay due to high carbon monoxide readings in the cabin.

Yeah, I don't like it when the flight crew slips into a coma midflight so I see this as a valid delay.
I ask the gate agent if he would endorse my ticket over to UA. Without any hassles whatsoever, he does it quickly and I literally run from B terminal to the mobile lounge and over to C. I get to the UA gate literally seconds after she closed the door. I ask if she would let me on as I have beads of sweat running down my face and I am panting. She objects at first and scolds me for screwing up her paperwork but relents after I mentioned 1K status and the fact the door was closed awfully early. Once I'm aboard, told to take an open seat. A few minutes later, Ms. Bad Attitude comes on board and says DL never endorsed the ticket and to get off. She does this loudly as I notice every other conversation around me stops in mid-sentence. I protested mildly but realize that if DL screwed up, I had no recourse and it wasn't right to delay everyone else. I removed myself and headed up the jetway with Ms. Bad Attitude walking briskly ahead of me shaking her head and muttering.
I hustled back to DL in the B Terminal. This time, the ERJ's door is closed and the jetway has been retracted. The same GA that moved me over to UA looked confused as to why I was back and then APOLOGIZED AND ADMITTED HE SCREWED UP as he placed me on a 10am-ish UA flight not expecting me to run to catch the earlier UA flight. He admitted to never telling me *which* UA flight he placed me on. As soon as he apologized to me, he radioed the Captain and said "hold on, one more!" Escorted me down the jetway and had me stand back as he maneuvered it back to the plane.
The surprised flight attendant must have thought I was some sort of special VIP.
I got to tell you, the DL agent was a helluva lot more professional and human-acting than the witch over at UA.
One gold star for Delta! ^