UK Post Office MasterCard is quite a money saver if a frequent overseas traveller though as it does currency conversion at market mid-rate and no fees. I assumed there must be a catch in their wording but after checking a few non-trivial transactions they were within pennies of spot rate for the day.

I think OPs issue arises from hotels having a mechanism / discretion to bill for items that appear after you depart such as late breakfast that didn't make the invoice, mini-bar binge that you didn't declare, missing gown/ pillow, etc. They then managed to put someone else's charged through accidentally.

If I picked up an extra charge somewhere I had been at rather that somewhere I hadn't I would assume cock up rather than fraud. I think Bank of Ireland (who run the PO card) should have done as well.