This one is not really a Barcelona Scam and is practiced all over the world - I once saw it on the back of a bus in New Orleans with 3 draughts pieces, a draughts board and a pea.
The one in Barcelona had disappeared for a while and the local council even went as far to hang notices up on the lamp posts on Las Ramblas with "It's a trick!" and an image of 3 cups and a ball. However, walking recently through town and I saw the unmistakeable crowd with the usual suspects. This scam invloves various people, although it's not necessarily apparent at the time.
One guy is on the floor with the usual follow the ball/pea, and there are a few others who step on the box and appear to be winning. These are the ones in on the scam, and they usually convince the "mark" to have a go - it's easy! Once the mark falls for the trick, they usually offer them a double or quits option, which again is more often than not taken and the game ends. However, recently I've seen a variation on this move, and foreign bank notes are used to make the mark "win". I have seen the guys with a 50 Soles from Peru held in such a way that it looks exactly like a 50 Euros note. There are a few look outs above and below where the scam is happening, in case the police arrive and they are also easy to spot. Next time you're near to the group, just take a step back and observe for a few minutes, it's really easy to see the ones in on it, and the look outs. I sometimes want to interrupt and tell people "Don't do it!" but some of the guys are huge!
The lamp post warnings came down and the scammers came back. The annoying thing is that according to the civil code, if they have less than 300 Euros on them it's only considered a minor occurance by the police and they are given a warning! If they have 400 Euros (almost all of them), they receive a fine. Easy, they pay the fine and continue when the Police have gone.
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