When Amy is scolding Laurie on New Year's Eve, Laurie's tie goes from only hanging on one side of his collar to hanging evenly on both sides of his collar.
When Friedrich is reading Jo's stories and offering his critiques, he alternates between holding a newspaper clipping and a book, depending on the camera angle.
When Amy is sitting at Aunt March's feet, one of Amy's braids alternates between being on her shoulder or behind her shoulder depending on the camera angle, before Aunt March actually moves the braid over Amy's shoulder.
At about 42 minutes into the movie, when the sisters are in the attic performing a play with Laurie, Amy's blue dress strap moves down and up with different camera angles.
At about 17 minutes in when Laurie first visit to the house, he takes off his glove twice. First, he takes it off at the door, then when Marmee urges him to come in he still wears it and takes it off again.
At 2:00:54 one can see Laurie (Chalamet) mouthes 'You love him' at the same time Amy (Florence Pugh) says the words to her sister Jo (Saoirse Ronan)
A plastic water bottle and Hydro Flask appear in the Laurences' study.
At the party at the beginning of the movie the music played was Waltz no.15 op. 39 by Johannes Brahms, which was published in 1866. The party was held during the Civil War, which ended in 1865.
Amy uses the term "marry rich," which did not exist in 1864. The term used would have been "marry well," which appears several times in Louisa May Alcott's text.
(at approx. 01:09:50) When Beth says "...and before anyone knew or paid you" to Jo, her mouth is not moving.
Amy gives up painting because she will never be a genius, so Laurie tries to console her by remarking that women aren't allowed into the club of geniuses, anyway - to which Amy retorts "The Brontës", overlooking that Anne Brontë, Emily Brontë, and Charlotte Brontë had to publish their novels as the brothers Acton, Ellis, and Currer Bell.
Saoirse Ronan's natural Irish accent is sometimes momentarily audible when Jo is angry and shouting.