Character mistake: In the Western scenes, when the characters are wearing spurs, they have the spur straps buckled on the wrong side (inside) of the foot. Spur strap buckles are supposed to be on the outside, not the inside, of the foot. Since it's wrong in every scene, it might be a comment on the TV show itself having low attention to detail.
Once Upon a Time in Hollywood (2019)
1 character mistake
Directed by: Quentin Tarantino
Starring: Brad Pitt, Leonardo DiCaprio, Timothy Olyphant, Margot Robbie, Austin Butler, Maya Hawke, Sydney Sweeney
Continuity mistake: During his conversation with the director, while discussing his costuming, Dalton's hair changes from messy and curled to gelled up and straight.
Cliff Booth: Hey! You're Rick fucking Dalton, don't you forget it.
More trivia for Once Upon a Time in Hollywood
Question: Was it intentional that Joe Namath and Ann Margaret's names were each repeated twice on the screen as Sharon Tate walked into the theatre? (01:08:00)
Answer: Yes, though the intention was not Tarantino's. What is onscreen is the trailer of the (real) 1970 film "C.C. and Company", which starred (as you might have guessed) Joe Namath and Ann-Margret. Said trailer showed their names twice, a stylistic choice that emphasized the film's stars, and Tarantino used the unaltered trailer in the film.