Someone approaches and 'finds' a gold ring near you, offering it to you 'because they don't need it.' It's brass. Walk away.
Other scams: a stranger 'spills' coffee on you, then helps you clean while picking your pocket. A child with a clipboard asking you to sign a petition — also pickpocket setup.
Best defense: walk briskly, don't engage. Polite refusal ('non merci, désolé') and keep moving.