People do this all the time, expect you to be psychic, I've learnt just to ask everybody who doesn't say if they have petrol or not, far easier and leads to less mistakes.
I've changed money up for people and given them a note for the change only for them to say I haven't given it to them, I just say oh I'm sure I did, they rumage round for a minute or two unable to find it. Ok then I'll just check the CCTV as it covers both tills and will show if I gave you the money or not, sure enough 20 seconds after I went outsisde he "found" the money :/ people try anything.
Also the people where you go, "That'll be £40 please" they put there card in pay and then walk out only for it to be the wrong pump, and there's is significantly more, I could understand if I didn't actually tell you the amount. People seem far more likely to speak up if it is the other way round.