When Topper gets ready to head for work after having breakfast with Mrs. Topper, he opens the entrance door with his left hand while holding his cane in his right hand. On the next cut as he exists the door, he is holding the cane in his left hand.
Near the beginning, George and Marion walk into the Rainbow Club. When Marion sits down at a table, she says "Thank you, Harry" to the waiter. Her comment has sometimes been mistaken for one of the restaurant patrons calling George by his real-life name, Cary.
When George and Marion appear and disappear, people and objects can be seen through.
When Topper is driving the Kerby's car, it can be see that a stunt man is doing the precision driving. At one point when Topper drives off the road to avoid hitting a bus, the driver has a full set of hair, whereas Topper is half-bald with a receding hairline.
In the scene where the invisible George is changing the tyre, the car is jacked up first and then the wheel nuts (lug nuts) are loosened. This cannot happen. If you try to loosen or tighten a wheel nut while the tyre is elevated, the wheel would just spin and you would not have the torque to do it.
Topper catching Constance from the Art Deco hotel cabinet was obviously a stuntman catching another (not as obvious) stuntman in a blonde wig. The 5' 6" Roland Young (aka, Topper) was supposed to catch the 5' 4'', 100-lbs Constance Bennett (aka, Marion Kerby.) However, Young's stuntman was stocky and had a full head of hair. Bennett's stuntman, in a wig with healthy bicycle rider calves, was more convincing.
(at around 9 mins) When Topper leaves the shower to get the towel held up by Wilkins, he can be seen to be wearing white boxers and isn't wet.
In the Sea Breeze Hotel Lobby, Topper has a chair follow behind him, forcing him to sit whenever he stops walking. The second time it happens, Roland Young (aka, Topper) anticipates it a good second before the chair contacts the back of his legs. This implies that Young was sitting without certainty the stalking chair was behind him.