Be very conscious...

I worked in this restaurant my first semester here. This guy came in - black, tall, looking kinda shady. He orders a salad, places his money on the counter (a $20 bill), saying out loud it was the smallest bill he had. Then he proceeds to make small talk with me. I hand him his salad and he stands there looking at me expectingly and I look back at him just the same. He then says, "My change?" And I reply, "You didn't give me the money." He gets very defensive and with his burst of emotion I suddenly wasn't sure if I did take the money up. I then said, "Oh did I? Ok" and I proceeded to give him his $15 change. As soon as he walk out that front door I realized what had happened. He ensured that I did in fact see the $20 by commenting about it and ordered the cheapest thing so that his profit would be more.
He left with his $20 and my $15!

The moral:
Be very conscious, these people are skilled. Even though I did find him suspicious I still wasnt sure if i picked up that $20. But 20 secs later, I was sure I didn't!