Just remembered a funny one - not my friends - my OHs.
Back in their younger day there were 4 of them who used to go out on a saturday night together. All lived near each other so night out should have been roughly the same in terms of costs, rounds bought, nightclub paid for, taxi fare home for all 4 split equally - they would all get out at the same place and just walk the last bit each.
Anyway, at some point he happened to say to one of the lads, 'wow, I seemed to spend a fortune last night, over 100 quid, how much did you spend?'. The other lad had spent a similar amount, but when the same question was casually posed to the other lads it turned out they had both only spent 60 or 70 quid.
My OH and the first guy got suspicious and began watching the situation more closely. What they discovered was that although the other two lads would stump up for rounds early on in the night, they would become less forthcoming later on, at which point there was enough drink imbibed that it was difficult to remember whose round it really was, so himself and the first guy always seemed to be the ones going to the bar (their own faults for not noticing!!). Then they noticed that one of the other guys would pretty frequently say things like 'Ive only a 20 left, rather than break it can I pay you back for the taxi/chipper/cigarette machine next week?' and it being so late and them pretty well on, all would be forgotten.
They worked out that over a 2 or 3 year period the two of them had subsidised the other two lads nights out to the tune of a couple of grand a year between them.
Now THATS scabby friends!!