If this was money in your bank account. and you spent it knowing or suspecting you were not entitled to it, you would be required to repay the money and committing a crime under the Theft Act 1968. In fact the burden is the other way round, you have to prove you had a reasonable expectation that you were entitled to it (and that seldom holds up in court). So probably best not to make an extravagant F booking as I suspect the same may apply to points (does anyone know for sure?).

To be fair, if I notice someone has given me too much change in a shop I tell them rather than pocket the cash. I know there are some who think I'm a sucker because of this.