(at around 1h 12 mins) When the Tramp and the Millionaire are drinking champagne with the burglars in the house, the Tramp places the glasses on the table behind them. When one of the burglar come up behind them the glasses are gone.
(at around 39 mins) When the Tramp accidentally swallows the whistle, his real eyebrows are visible beneath his fake ones. This was done intentionally to give him a lopsided, intoxicated facial expression.
In the famous fight scene, several spectators in the back row(s) of the gallery are motionless, either painted on a matte painting, or cut-outs or mannequins. In the shot of The Tramp dreaming of the flower girl, and the following shot with the trainers again kneeling around him, we see a particularly obvious mannequin or statue, a man in a light gray suit, in the lower right corner of the screen. He also is visible in other scenes.
(at around 1h 9 mins) When the Tramp is knocked out on the table in the locker room you see a pair of boxing gloves hooked on a post behind the table. The Tramp wakes up and struggles to sit up. The entire scene you can clearly see a wire attached to one of those gloves that when triggered, falls on his head, knocking him out once again.