Nervous Man in a Four Dollar Room - S2-E3
Revealing mistake: The main character's reflection in a mirror has come to life and much of the episode is a confrontation between the character and his reflection. Although they made an effort to reverse the image (the reflection wears his watch on the opposite hand and he bites his fingernails with the opposite hand) they forgot to reverse the buttons on his shirt.
Revealing mistake: When "Selden" falls down the stairs it is obvious that it's a stunt double.
The Jungle - S3-E12
Revealing mistake: The "mannequin" dressed as a warrior in the costume shop window is a live actor; watch closely and you can see his eyes widen ever so slightly. (00:14:55)
I Shot an Arrow into the Air - S1-E15
Revealing mistake: Right after their friend dies and the commander talks about going into the mountains to search for any life signs, you can very briefly see Corry looking straight into the camera.
Revealing mistake: When Lanser drops the cup, you can quite see that he is drinking from an empty cup, since the steward never poured him any and yet he manages to get some of it on his suit.
A Nice Place to Visit - S1-E28
Revealing mistake: When Rocky evicts three women from the room and slams the door behind them, you can quite clearly see the entire wall shaking.
And When the Sky Was Opened - S1-E11
Revealing mistake: After his flashbacks in his hospital room, Forbes looks at himself in The Mirror and he cannot see his reflection. But you can still see his arm in mirror.
Revealing mistake: In scene just after Rod Serling's opening narration, you can quite see that scene is being played in reverse because smoke is coming downwards back into the man's cigarette.
A Nice Place to Visit - S1-E28
Revealing mistake: After Valentine finds out he isn't in Heaven, he tries frantically to open the door. As he does, you can see the whole wall and door frame moving along with the door, showing that it's fake.
The Eye of the Beholder - S2-E6
Revealing mistake: In the episode, all the people are shown from the neck down to hide their faces (which at the end we see this is on another planet as they are alien looking). But as the first nurse seen in the episode walks to the window and turns around, even in shadow you can see that she is normal looking, that is, no prosthetic or make-up applied for the alien effect.
Revealing mistake: When the old lady touches the door handle right before getting cut by the knife she gets paint smeared on her hand from the door handle before being cut with the knife.
Nervous Man in a Four Dollar Room - S2-E3
Revealing mistake: When Jackie pulls the wardrobe from the wall and spins it around, The Mirror briefly reflects an area beyond where the set ends.
Revealing mistake: When the main character proclaims, ".the new Walter Bedeker!" you can see the frames are running back and forth. You can tell by looking at the window curtains behind him.
Revealing mistake: One of the actresses who is supposed to be playing a mannequin doesn't start to smile until the camera is on her.
Stopover in a Quiet Town - S5-E30
Revealing mistake: The actors are supposed to be in a play set for the huge child, but when she reaches down and picks them up it is obvious that it was filmed in an outdoor setting in a town much bigger than what is supposed to be the play set.
Answer: It's explained in the episode. The aliens are experimenting to see how humans react to various scenarios. This just happens to involve them using super strength.
Thanks. It's been a long time since I saw the episode so I forgot.