There was a period where I would always get free drinks by pulling a used cup out of the trash and getting a free refill. The trick also worked for buckets of popcorn pulled out of the trash at the movie theatre, which made for great free times after sneaking in without paying in the first place. Humans are equipped with this wonderful thing called an immune system, which protects them from nasty communicable illnesses, and I never got sick doing this. I no longer advocate such behavior, though, or even partake myself, out of fear of finally catching something from the practice. If I knew what my odds were, I'd probably still be doing it, but ignorance can be terrifying. In truth, it's probably almost as safe as eating in any restaurant. You don't know where the people making your food have been any more than you know where the person who threw away their cup has been.
Once, I was waiting in line at Taco Bell when I decided to show my friend how it was done. I went to the trash, pulled out a cup, got a refill, and got back in line.
"See that? Free drink!" I said.
"That lady is telling on you," he said, and pointed behind me. A woman was sitting with her teenage daughter, pointing me out to an employee with a broom and looking completely disgusted and offended. The employee looked at me, and then started walking in my direction. I wondered what she was going to say.
"Excuse me," she said. I moved out of the way, and she walked passed me. Apparently, there was no policy against doing that. When I ordered my food, I placed the cup conspicuously in front of me on the counter, but they didn't have anything to say about it, either.
When we left, we stuck it to The Man by taking all of the Fire Sauce.