Alas, we got caught with one, as I was only looking at the total and hadn't spotted the $180 in payments that had already been taken. The dollar figure seemed about right but found after disembarking that Barb's purchases came across to my folio for payment on my credit card, but her OBC didn't and she disembarked with her $150 non-refundable OBC intact. That could be a FCC for 15 eggs bennies, but P&O said bad luck. In return, it has cost them far more than $150 already.
So, it is more than annoying if you are having a good time and just looking at your outstanding balance. In balance though, theses are actual card charges and not holds, so there should be no lingering holds if they do it that way.