Monkey trainers will entertain you with their animals and will let you take pictures with them. However, as funny as this might sound, some monkeys are trained to pick your wallet from the pocket or purse.
The scam is not in getting the monkey to do it, as you will probably catch her (or not), but in the reward that you are actually asked for, as tip: “I am sure you don’t want the monkey to run away with it now, do you?”.
How to avoid: whenever traveling, always keep your wallet in a pocket or place that is zipped up.