This one invloves a few scammers, an empty car parking space, and a tourist in a car - the locals don't fall for this one. Like the flat tyre scam, this involves theft of something left visible on the back seat, on the dashboard, or on the back shelf, for example. Here's how the scam unfolds - I've seen it happen in broad daylight, just near the bottom of Las Ramblas.
A car approaches a free car parking space, and lines up to reverse park into the space. There may or may not be a few people hanging around on the path/shop fronts nearby. As soon as the car reverses in, an angry member of the public approaches the driver, to complain that they've just bumped their car/stolen their parking space. Of course this hasn't happened, and any driver will know that you can almost always tell if you've hit another car whilst reversing into a space and that even if you did it would be bumper to bumper, so shouldn't cause much damage. As the person is so insistant, you get out the car to either confront the person or see what they are jabbering on about. As the door opens, the accomplice takes whatever it was you left on view. In the confusion, the victim often is unsure that the scammers are together, and usually gives chase to the theif - giving the perfect window for the angry protestor to slip away.
The same rule applies - if you have nothing on show, then this scam will probably not happen to you. If it's impossible to have everything squashed in the boot/trunk, then just try and keep your less valuable things, or things like food you might have in the car on view, rather than say the laptop or portable DVD! And if anyone does come up to you angrily, claiming you have taken their space, drive off. If they claim you've bumped them, then be aware if you need to step out to check.
For more information on Barcelona, see this Barcelona Guide.
Barcelona apartments for groups