OK, this is timely. This story, literally, happened to me this evening in Sandton City Mall in Johannesburg. To me, it was a clearly blatant scam. But my colleague was second-guessing himself . . .

We stopped by a bank ATM in the mall to get
some cash. I finished my transaction, got my card and receipt. Some well-dressed guy comes up to me and starts pointing to the ATM saying
my transaction was not finished and I needed to put my card back in to
clear the system. I told him my transaction was completed and I had my
receipt. He kept insisting that it was not and that he couldn't use the
ATM unless I closed my session. I told him I was NOT putting my card
back into the ATM machine so he went over to the other guy I was with
and tried to get him to do it. As it turned out, he had not entered his
pin correctly and thought there might be something to this. But I
convinced him not to try it again. I told the scammer that if the
machines were hung up, the lady I could see inside the bank would help
him and he said that they were closed. I told him that wasn't my problem
and we walked away. I never really felt threatened by the guy and I was
bigger than he was.

I mentioned the incident to the proprietor of a restaurant a few minutes later. He got pissed off and had one of his waiters go out and call security to report what had happened. 10 minutes later we got there and explained what had happened. They said that this happens a lot and the scam is for you to hand your card to the guy so that he can show you what he's talking about. They then palm your card and give you a fake one. Now, I think I would have noticed if I'd given him my card that he didn't give me MY card back, but I guess once they get hold of it they just take off.

Trust your instincts. If it doesn't feel right, WALK AWAY.