Continuity mistake: Before Pollyanna arrives on the train, when Jimmy Bean pushes the metal hoop down the road, two girls stop their doll carriages right beside each other and grab their dolls, but in the next shot their carriages are about 4 feet apart, which is now wide enough for Jimmy to run between. (00:01:10)
Factual error: Jack Lemmon cooks spaghetti for Xmas dinner. On New Years Eve he picks up the tennis racket, through which he had poured the spaghetti, and one piece hangs limp from the racket. It would of course, have long dried and be hard as before cooking.
Continuity mistake: When Cary Grant tells Deborah Kerr to keep the door closed, she is holding a record album. As a reaction to his directive, she lowers the LP to about waist level. The camera shot changes and she's holding the LP higher again. (00:27:55)
Visible crew/equipment: When the slaves roll the flaming ball down the hill, we see it strike several Roman soldiers. Look closely, they are actually stuntmen dressed in asbestos suits.
Character mistake: When Dean Martin desribes his writer's block to Judy Holiday, she says that she keeps sharpening his Ticonderoga pencil until there's nothing left but the "Ti," but in fact what would be left on a little nib is the "ga."
Continuity mistake: In the bar scene, near the beginning of the film, the pint glass that Albert Finney holds, constantly changes position. (00:06:05)
Audio problem: When Merritt is arguing with her professor, she says, "We're supposed to be intelligent" but her lips are actually saying "Don't be afraid; my opinion is yes." This is the exact same shot used a moment later when she actually says this line. (00:07:10)