Recently, at a restaurant, a friend and I had an amazingly long wait for what should have been fairly simple food. The place was not particularly busy, so it made the wait all the more egregious. When the waiter stopped by to apologize for the long wait, I asked about our appetizer. He laughed and said "I'm sorry I forgot to even put that part of the order in."
We'd come in very hungry - it was a very late-in-the-day lunch and we'd had early breakfasts. Our blood sugar was crashing at that point, and we'd stopped in this restaurant based solely on the fact that it was right in front of us. Needless to say, we weren't laughing. Our appetizer actually arrived AFTER we'd finished our meals.
The waiter took 50 percent off the bill, which was nice, I guess. It was my friend's turn to pay for lunch, so I didn't pay attention to the tip - he said he didn't leave a very nice one though. How much would you have left? I normally tip 20 percent for decent service, and rarely less than that. But honestly, I've never had an experience like that.