CELLOS ON THE MOTHERFUCKING PLANE

Mrs. A isn't a pro but she has traveled her cello a few times. There's a good chance the people at the counter don't know what the policy/routine is, and because every airline handles it differently - puts it in the computer differently - booking through codeshares is a problem. Isserlis, another famous cellist, is always complaining on twitter about his travel problems. You'd think "I am a fancy professional cellist going to play at this big concert hall" would overwhelm some resistance, however...

Famed British cellist Sheku Kanneh-Mason, who became a household name after performing at the wedding of Prince Harry and Meghan Markle, has said he had to cancel a concert in Canada after the country’s largest airline denied his pre-booked seat for his cello.

...

“First we had delays, then a cancellation, and the day concluded by being denied boarding with the cello – despite having a confirmed seat for it – on a new, final flight into Toronto. After nine anxious hours at the airport, we realised our journey wasn’t going to be possible,” the statement continued.

“We can only dream of a time when all airlines have a standardised, global and carefully considered approach to the carriage of precious instruments that are booked to travel in the cabin,” they added.

Based on our experiences: some airlines code it as a seat with a name like "A. Cello." Some code it as extra cabin baggage. If it's coded as baggage they will bump it for a person.  If the computer has done that, and it's a full flight...

I've seen Sheku a couple of times.  Talented and charismatic performer.