This is actually quite a serious post and one I read up on a well respected travel forum. All my previous posts about scams have involved getting duped or pickpocketed, but unfortunately this one involves violence.
The marks were 4 girls in their 30's and the scam takes place last New Years Eve in the Olympic Village, so I know for a kick off this is a very specific scam to be writing about, and not necessarily something you think might happen all the time, but I still think it's worthy of a heads up.
The scam unfolds in a restaurant (in this case a restaurant in the Olympic Village which I won't name) and when the bill is brought, a credit card is given. Don't get ahead of yourselves thinking it's a card cloning. The waiter takes it away and returns with the bill which has been overcharged by 100 Euros. So they ask you to sign, apologise and give you the 100 Euros in cash - their mistake. The girls leave the restaurant with 100 Euros cash that they wouldn't ordinarily have, and have their bags snatched very close by - but only 3 of them.
So the victim who posted the warning suspects that the waiter is the instigator of the scam, and then communicates their description via a mobile (or whatever) to his cronies outside who then lift the bags - this is why they only take 3. Maybe 3 were brunettes and the other a blonde, so they knew it wasn't her for example.
How to avoid it? If you're ever overcharged, then insist on the amount being credited back onto your credit card, and make sure you get the receipt to verify that. Most restaurants have battery powered TPV machines for credit card transactions (chip and pin is not huge in Barcelona) so the machine could be brought to your table and swiped.
For planning your trip against any possible mishaps, see this detailed Barcelona Guide and check out my other recent posts on Barcelona Scams:
The School Questionnaire
The Postcard cover up
The Reverse Parking Scam
3 Cups Scam
The Map Readers Scam
The Football Dance Scam
The Flat Tyre Scam
The Ketchup Squirt Scam
The Dancing Dollies Scam
For accomodation in the city, try these apartments for rent in Barcelona or if you're travelling with a family look at these apartments in Barcelona for families.