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: 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)
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)
Life's Ruff - S1-E20
Character mistake: When first showing the Florida State Poochitentiary, the white sign in front of it misspells the name as "Florida State Poochitentary." (00:06:41)
Character mistake: Jessie's dance moves don't match what the other girls are performing, even though it's a choreographed routine.