1. The people who demand the most service ("This is too cold, take it back", "Bring me a different fork, this one has a spot on it", etc) tip significantly below average, and sometimes not at all. This pattern is present just about everywhere- I see it in IT all the time.
2. Groups, especially business meetings, are the highest tippers. Serving a table of 8 isn't significantly harder than serving a table of 2, yet groups usually still tip based on percentage.
3. People who use coupons are universally small tippers. I also suspect that folks with coupons are not likely to be return customers.