NEW YORK TIMESBy Christine NegroniOctober 28, 2009 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.