Peas on a Knife

"In 1848, Thackeray's Book of Snobs used that disgusting pea practice as a paradigm: 'Society having ordained certain customs, men are bound to obey the law of society, and conform to its harmless orders.' If I were to eat my peas with my knife, I would be 'insulting society' just as much as if I were to go to a tea party in a dressing gown and slippers. Homer Simpson eats his peas with a knife, and that says it all." (68)

Pegnoir, c. 1900, in the Goldstein Museum at UMN.
From a great post on their blog here.
Shapin, Steve. "The Tines They are A-Changin': A history of table technology" [review]. Harper's Magazine January 2013.

Here, dress codes are considered the most applicable comparison of rude behavior. I like this.