(ETA: I want to clarify something: I had no idea until R.A. posted a comment that "porch monkey" is used as a racial slur. I've never heard it before--I just called bitchy, ridiculous people on the patio "patio monkeys" until one day I accidentally called it the porch, and it stuck. I in no way meant anything racist by it--they were upper class whites, in fact. I apologize if anyone was offended.)
$2 in quarters is never a great tip. But on a $68 bill, it's an insult. On perfect service, it's an insult. When you sat on the patio and were annoying little patio monkeys the entire time, it's an insult.
And when all those circumstances combine, somebody is a big fucking bitch.