It wasn't BA but... I recently went in an actual shop and made a purchase for about £600. Happened to look at the pending transactions on my Amex card the next day and it showed 3 separate entries for that purchase as pending. I hadn't seen this before and had a bit of a panic but decided, as it was an actual shop, I could always go in and make a fuss if necessary. Few days later there was one actual charge and the others had disappeared. So I think this must be a feature of the way the credit card magic works.

I also agree about complaining if it was the phantom transaction that pushed you over the limit. You shouldn't be penalised because of a hidden feature of the system.