When Hal hops a freight train to Tulsa, he jumps onto a boxcar that has its door shut. He climbs the ladder and stands on top to wave to Madge. In this long shot, the boxcar door is now open.
When Rosemary forces Hal to dance with her on the pier, the flowery white earrings she's been wearing disappear. In the next shot, when she presses her head against his cheek, the earrings reappear, one scratches her face, and she apologizes and removes both earrings.
During the picnic while Hal is carrying Millie above his head with one arm, her knees are exposed. The next angle scene shows Millie holding her dress below her knees.
Hal's hairstyle changes during the pondside confession with Madge.
Millie's hair is wet and braided after swimming; later in the same scene, her hair is dry and braided.
Millie is reading Carson McCullers' "The Ballad of the Sad Cafe", a very thick book she carries around early in the movie. In reality, that literary work is a very slim novella of under 100 pages. However, the original edition of that book was bundled with McCullers' novels, so it was a thick volume.
When Madge is in the swan boat, she says "Hi" to Hal, but her lips do not move.
When Hal arrives at Howard's house after running from the police, Howard greets him as "Allen" instead of Hal, as he runs up the stairs.
Judging from the position of the sun and the bustle of activity in the neighborhood, Boomer is delivering the morning paper (an edition which typically arrives around dawn) shortly before noon.