According to this newspaper story, a woman in Ohio found a tooth in her pizza! I'm skeptical. I've worked in the pizza business for 4 years, and nobody I've worked with ever would have messed with a customer's pizza.
And if somebody was going to do something, why would they leave something so easily found (and traced) as a tooth!?!?!
Now, if you're a repeat stiffer, you might get your 2-liter well shaken, but foreign objects on the pizza is crossing a line that nobody I know would think of crossing.