As Mary and Aunt Paula exit the room after telling Jeff that Mary is to be married, he takes out his cigarette case and opens it. The scene cuts to Shep coming in and back to Jeff who suddenly doesn't have the case out and is leaning against a table.
When Dill and Mary are driving in the Rolls Royce, the camera pans up from the grill, and the "Spirit of Ecstasy" hood ornament is not there. However, moments later when the camera view changes to the car interior, the ornament can be seen through the windshield.
Shep exits the taxi with many more balloons than which he entered it.
Dill and Mary fall while running in the rain, leaving her skirt muddy and soaked from her hip to her knee. In the next scene at the door, the mud stain is gone and upon entering the room her clothes are completely dry.
At Dill and Connie's party, Jeff takes out his cigarette case, removes a cigarette, and returns the case to his right inside coat pocket. When Dill meets him, Jeff is still holding the cigarette case and again puts it in his right inside coat pocket.
When Mary shows Jeff a picture of the three of them as kids, it's a photo of children with the three stars' adult pictures obviously pasted on their bodies - Mary even has on lipstick. Though they did at least use a picture of Clark Gable without a mustache.
When Robert Montgomery exits the bathroom singing "The Toreador Song", he starts singing before his mouth opens.