Being really scatty when it comes to my own finances (training to be an accountant and when it's other people's money I am very organised and methodical) I'd be one of those they'd catch out and I really wouldn't remember.

As such I make sure, when viable to pay by card, that way you've got clear evidence of what has/has not gone out of your account.

I agree with others that it's probably not worth losing a good T due to an admin cock up... So yeah view it as a lesson learnt and put it in terms of 'they aren't going to do it to me again' rather than stewing on something that's done and could be stressful to argue over?