Customs of tipping

Today at work, we had an explosion of people. It got so busy that I actually forgot about one of my tables. 100% forgot. I put in their order and then didn't talk to them again. My manager at one point asked me if the table was mine, and I told her no. I didn't bring them their food or refill their drinks or say goodnight or anything (I did make them some killer hot chocolate, though.)

They left me a 20% tip.

When I realized what I had done, and what they left me, I was so upset. I felt like the worst waitress ever, wanted to cry and tried to give the five bucks to my manager, who had been taking care of the table (she refused.) How does stuff like this even happen?

Moreover, I have come to the conclusion that everyone has their own opinions regarding tips. People, I've decided, learn at a young age what is appropriate to tip (probably from their parents) and stick to that, and it will apparently take a lot to change their mind. I'll get some tables that, no matter how stellar my service is, they don't feel they need to leave me more than 3%. On the other hand, I'll get people like I had today, who (other than the killer Hot Chocolate) got the worst service ever, and they still left me a generous tip.

Then I thought about how I've always been a firm believer that it's necessary to leave a solid 15% tip, and what it would take for me to leave less. I could only come up with two things that would cut a tip: 1) If the server was rude and 2) If the server was ignoring me, and very clearly doing nothing productive (ie. "I need a refill and she's been talking with the hostess for 20 minutes," not "I need a refill and she's tornadoing around the restaurant trying to maintain her other 7 tables.") I would never penalize a server for being too busy, or if my food was made wrong, but I know a number of people who would.

The custom of tipping is interesting to me, mostly because it seems like such a cut-and-dry issue and yet people get so worked up and set in their ways regarding it.