United Airlines learned its lesson the hard way that David Carroll was not just another customer.

After baggage handlers at United broke his guitar last summer and the airline refused to pay for the $1,200 repair, Mr. Carroll, a Canadian singer, created a music video titled “United Breaks Guitars” that has been viewed more than 5.8 million times. United executives met with him and promised to do better.

So how was Mr. Carroll’s most recent flight on United?

This Everyman symbol of the aggrieved traveler was treated, well, like just another customer. United lost his bag.

In an interview, Mr. Carroll said that for more than an hour on Sunday, he was told he could not leave the international baggage claim area at Denver International Airport, where he had flown from Saskatchewan. He said he had been told to stay because his bag was delayed, not lost, and he had to be there to claim it when it came down the conveyor belt.

“I’m the only person pacing around this room,” Mr. Carroll said, recalling how he was caught between an order from United staff members to stay and collect his bag, and a federal customs official telling him he had to leave the baggage claim area. The bag never showed.