I know someone who had £1800 taken off his MBNA card which wasn't his transaction. He phoned them up immediately and they said the transaction had been made by one of their cheques and they would investigate it, but he would have to pay £25 admin charge.
He said he had never used their cheques and someone must have intercepted the post and used the one they sent out with a promotion. They seemed quite disbelieving, so he went to the police and got allocated a crime number. Eventually, he had a phone call from the police to say that there was no crime - it was the MBNA bank's computer made a mistake

and that the money had been refunded. MBNA themselves didn't have the decency to phone him and tell him that - he had to find out from the police!
He then wrote to MBNA and asked them to refund the £25 admin charge and some extra compensation for all the hassle and time it had taken up. I'm not sure what the final outcome was, but I know he's not with them now!