Nothing, but it was crap service.
Nothing with a note that said; "I looked for you all through the meal, now you look for the tip"
Nothing, but it was by accident. Someone had spilled a bunch of bleach in the restaurant's kitchen and as I am sensitive to chemical smells I left in a rush, only realizing later that I had forgotten the waiter!
I've left, "No Tip" countless times when the food and service were extremely lacking. I know of someone that has added in the tip line, "Tip: Get a job that you're good at!" It's kind of harsh. Yet, it is better to be real than to politely lie to someone. How will they ever know what they're doing wrong?
Nothing. It's so easy to do.
1 cent with an explanation note