Man, I remember a long long time ago I was at Burger King with a few of my friends.
We were discussing whether or not one of us could pull this off. My friend ordered a meal, ate all the fries and most of the burger then ripped out some hair from his head and put it in what was left of the burger.
He showed it to the guys at the counter and they were like "damn, we're really sorry" and they gave him another meal for free.
I've never done it myself and only seen it done once but it seems its not that hard to pull that scam off.