It's a common problem, seen it many times with different bunches of people.
Thurs night I was out with a bunch of guys from work. I got the first round as I always try to (for eight guys, wasn't cheap), later a few other guys got rounds in, then after about four rounds some people are standing there with empty glasses.
So I ask the guy I arrived there with what he wants, and he knows damn well that I already bought the first round - and that he hasn't got one, and he's like "Yeah cheers, same again." So, I'm buying another round already!
It gets a bit annoying but I'm not tight enough to worry about it or let spoil my evening. The funny thing is, the "Yeah cheers, same again" guy is a higher pay grade than me and is always bragging about the houses he owns and how he could have any car he wants, even though he drives a diesel Seat.