Character mistake: When George and Elaine go to the coffee shop after The Movie without Jerry, they establish that Elaine moved to NYC in 1986. Then they make fun of the way that Jerry throws up; both imitating Jerry. The "Dinner Party" episode is after the "The Dog". In the "Dinner Party" episode, Jerry brags that he hasn't thrown up since June 29th, 1980. The black and white cookie ended this streak. Elaine would not have been around to see this, not arriving in NYC until 1986.
Character mistake: At the end of the dance routine, all of the girls strike a pose with one hand on their hip and the other behind their head, except for Stacey who just stands there.
Suggested correction: Stacey was in New York with her father for most of this episode, so perhaps she didn't learn the entire routine before performing it.
Stacey did the entire routine with the girls, even though she wasn't always on camera; it's suggested she knew the moves, so the fact she missed the end pose stands.
Toddler Terrors of Time Travel - S1-E8
Character mistake: This series is a continuation of "The Super Mario Bros. Super Show." However, in this episode, King Koopa implies that he has never taken over the Mushroom Kingdom. This contradicts a main plot point of "The Super Mario Bros. Super Show!", where in many episodes, Mario and his friends look for someone or something to help Princess Toadstool free her kingdom from King Koopa, who has taken over her kingdom.
Lady Florence Craye Arrives in New York (or, the Once and Future Ex) - S4-E2
Character mistake: Cheesewright is trying to convince the police to raid the charity ball because alcohol is going to be served. The police chief tells him they stay out of private affairs and Cheesewright replies "The law's the law!" Prohibition made illegal the sale and transport of alcohol, but people were still allowed to have it in their homes or private functions. No laws were being broken. (00:33:30)
Character mistake: When Geoffrey serves tea after his sassy cough at Vivian's sentence about High Tea not being a sacred institution, the actor messes up when it's time to actually serve tea, with a napkin getting caught in the teacup saucer. Granted, accidents do happen, but it's quite uncharacteristic for the perfect butler Geoffrey is portrayed to be, and simply a flub by Joseph Marcell. (00:17:45)