I always tip at least 20%. I know a lot of trouble is people on the grill, so I take that into account. Even if the waitress messes an order up, it's okay- everyone makes mistakes, and if she's polite about it, I'll tip her even more. It's karma.

I was at a bagel place today, they messed my order up 4 times. That's okay- I wasn't in a rush, nor starving, and my pleasantess was mirrored by the staff each time they brought a wrong order out. It happens. I left a 25% tip, and everyone was happy. In perspective, it's not that big of a deal. If you're waiting forever for you food, they get it wrong, you're pressed for time, it's cold, you're starving, yeah I can see getting upset about it. But if I'm in no rush and am treated with respect, you deserve the same amount of tip.