Arnold wears mid length socks/shorts at Mumsy's birthday party leaving about a foot of his legs exposed. Moments later when the horse jumps the table his stunt person wears much longer shorts leaving only his knee caps exposed. Also a waiter and waitress suddenly appear at the right side of the table.
Iris clearly does not slap or even mimic a slap of Arthur otherwise her arm would have been visible as the camera focuses on Stanley sitting at the bar.