This scam has been played on unsuspecting travelers in Buenos Aires:

You are approached by a couple who ask you to take their picture with their camera. When you take the picture, the so-called camera, you won't hear the "click" of the shutter, however, you will hear a "pumping action" instead as the camera squirts some type of sticky substance out of it, and onto your clothes (or, if you are holding the camera close, into your face or eyes!).

The couple will try to assist with the "clean up"... refuse this and walk away! You could easily be pickpocketed or mugged, etc.