A popular scam in Europe involves one or more people pretending to be deaf or mute. The scammers, often children, will approach tourists with a petition requesting donations to a charity, and begin aggressively asking you to give them money.
At best, you'll find yourself in an uncomfortable confrontation, but you also risk getting pickpocketed by members of the group if you linger too long. And for what it's worth, as the YouTube channel Honest Guide found out, the scammers are almost certainly not deaf.