I refuse to pay $$$ for something I didn't ask for.

I send it back. I used to be a server, so I make it a point to be polite to the server regardless of whether or not my order's correct. Even if you don't send it back, the server could easily be spitting the food, especially if the customer's being an asshole. It's easier to just be patient and nice.

Lots of restaurants have cameras inside the establishment now to monitor how the employees are performing.