When Mr. Mannix is listening to a tape, the amount of tape left on the reel is greater when he switches the player off than when he is listening to it.
When Mannix first approaches the photograph of his Army unit in Korea that is displayed on the wall, there is a grease pencil dangling in front of it on a string. Then the string and the pencil are not visible in the first closeup of the photograph, but in a subsequent closeup the shadow of the string and pencil hide part of the photograph, even though Mannix hasn't touched the string, pencil, or picture.
Both gas grenade shots use the same footage. Note the paint spatter on the floor where the grenades land.
In the closeups of Gus' hand holding a two-way radio, he doesn't press the talk button while talking to Mannix multiple times.