I had my pocket picked at school by a guy who evolved into a lifelong criminal. My wallet was in my back pocket and it wasn't all the way down so the top was exposed. He bumped into me as he did it and then created a distraction by looking at someone else with a look on his face that implied the other guy had pushed him into me.

So I didn't realise for about 5 minutes later when I saw him and his mates going through my wallet.

From that day on I'm always suspicious if someone bumps into me and instantly check that my wallet is still there and I've never had my pocket picked since.