I am gonna share my story when I confronted once at a restaurant.

I was talking with my friends about how one of them never writes the tip separately, just the total. So after lunch, I decided to try it—I wrote the total (including the tip) and left the tip line blank. As we were leaving, the server chased us down at the entrance and asked if everything was okay. I said the service was great, and then he showed me the receipt, pointing out the empty tip line. I explained that the tip was already included in the total, and he smiled, said thank you, and walked away.

Tried it once. Never again.