When Benjy and Swann are in the limo after meeting Benjy's family, the drink decanter and glass swap in Swann's hands between the first and second shot.
In the first scene with Swann--in bed, as he banters with the airline stewardess--his pack of Old Gold cigarettes leaps from his chest to the bed.
The hot dogs are shorter than the buns when the vendor smears mustard on them, however they are longer than the buns after he hands them to Alan and Benjy.
The film shows full dress rehearsals in the broadcast studio early in the week before the air day, complete with scenery. Rehearsals were done in a rehearsal space, so studios could be available for other shows on other days. Dress rehearsals with scenery in studio would be held on the day of the broadcast.
The show on before King Kaiser's is The Kate Smith Evening Hour (1951), but that program was only on from 1951 to 1952 - not 1954, the year of this film.
As Alan Swann (Peter O'Toole) and Benjy Stone (Mark Linn-Baker) are preparing to leave for dinner on their first night together, Swann says that, as a dramatic actor, he doesn't understand comedy and then quotes the great English actor Edmund Kean as having said on his deathbed, "Dying is easy. Comedy is hard." In fact, there is no evidence that Kean was the one responsible for that line. According to director George Seaton, character actor Edmund Gwenn is indeed supposed to have said something similar (though not quite as eloquent) on his deathbed, although Gwenn died in 1959, and, thus, could not have been quoted by Swann in a movie set in 1954.
In the movie, "The Comedy Cavalcade" is broadcast from an auditorium studio in 30 Rockefeller Plaza. In the 1950s, NBC had leased various Broadway theaters for their prime time variety shows, which were not done from 30 Rock. The 30 Rock studios were built to produce radio variety shows and were suitable for game shows and later programs like The Tonight Show (1953), but could not handle sketch comedy shows with large casts and lots of scenery. (Studio 8-H, now home of Saturday Night Live (1975), was set up for live dramas with no studio audience.)
Early in the movie, a labor leader and his attorney meet with the show's producer to state that an actor's portrayal of a person having that labor leader's character is "slander." The producer replies that the labor leader is a public figure, and so the test of defamation is more difficult than simple slander. That test, the Public Figure test, was not developed until ten years after the year the movie takes place. This is correct insofar as federal Constitutional law is concerned, in New York Times vs. Sullivan. However, the public figure test had been adopted by various state supreme courts well before the U.S. Supreme Court adopted it nationally, so it is not necessarily incorrect that a public figure would have a harder time proving slander, even in 1954.
When Benjy is clumsily carrying the dessert tray, at one point he tilts it at such an angle that the desserts should have slid off onto the floor. Yet, not one dessert even moved.
Benjy and K.C. have fifteen or twenty containers of dim sum in front of them during their "dinner and a show", but never eat any of it.
In the street scene following the "hot dog" shot, the block Benjy and Swann are walking in is a real NY street dressed for 1954. The next block behind them and the skyline, including the long-gone Astor Hotel, is a matte shot. Several modern buses and an RV can be seen under the marque over the left sidewalk.
[possibly deliberate] The cardboard cutout of Alan Swann as a young man is neither Peter O'Toole nor Errol Flynn, but Oreste Kirkop, who starred in the second film version of the swashbuckling operetta The Vagabond King (1956). This movie, however, happens in 1954.
When Swann is dangling off the side of K.C.'s parents' building, all the cars seen on the street below are 1970s and 80s automobiles, though the film is set in 1954.
The cameras in the Comedy Cavalcade studio are RCA TK-14s, which were not introduced until the late 1950s. (NBC would have used RCA TK-30, TK-10 or TK-11 cameras for black-and-white shows at this time.) In another scene, a Marconi Mark IV camera is being pushed through a hallway; this camera also was not available until the late 1950s.
When Benjy follows Alan Swann to his limo in front of Rockefeller Center just before the big climax, U.S. state flags flying in the background include Georgia's state seal/Confederate battle flag design, which wasn't adopted until 1956.