Deliberate mistake: Rob is about to leave his room, and he opens the bedroom door to the noise of a loud music box that was originally being made by Laura's music box that holds jewellery in it.
You Ought to Be in Pictures - S5-E28
Deliberate mistake: The movie director tells everyone "ACTION!" Yet Rob stands outside the door and doesn't come in, then he says "uh...action?" Then Rob pokes his head in and asks Leslie "Did you say something?" In "Somebody Has to Play Cleopatra" Rob is the director of the play. He says "Action!" and then Harry (Bob Crane) does not come in on cue several times. Rob also calls Leslie "Liz". (00:15:35 - 00:16:20)
Deliberate mistake: While Rob is on his honeymoon, as well as 3 day pass, things like that, Rob still wears his army outfit. Yet in real life us real humans wouldn't be wearing the same clothes every time we were out. We would go home and change.
What's in a Middle Name? - S2-E7
Deliberate mistake: Rob, Sally and Buddy's "old office" is exactly the same shape as Robs basement (except for the door in the center of the back wall. Instead of going upstairs it's all on 1 level. There's a door to the right which goes into a random room which we never get to see inside). (00:10:38)
Suggested correction: Since you listed multiple differences, it's obviously not "exactly the same." Filmmakers not building every set and prop from scratch is not a mistake. Modifying and reusing old sets is just an economic reality of production. If anything this should be under trivia.