McNuggets of Information

This is my latest at the Partial Observer. Excerpt:

A fast-food restaurant sells chicken nuggets in boxes of three sizes: 6-piece, 9-piece, and 20-piece . . . But one customer regularly comes in asking not for these particular sizes, but for a total number of nuggets that no combination of these box sizes can fill to the exact amount. When the customer sees he will be charged for two 6-pieces when he orders ten nuggets, or one 20-piece when he orders nineteen nuggets, he gets angry and leaves. The question: what is the largest number of nuggets the customer can order, that the restaurant can't fill to the exact amount?