Visible crew/equipment: As Ferris is standing in front of the bakery, you can see the reflection of several crew members in the window. You can also see the reflection of another crew member on the glass inside the phone booth.
Visible crew/equipment: As Henry is walking down the stairs to the bank vault, the camera's shadow is visible on the side of the staircase. (00:09:45)
The Four of Us are Dying - S1-E13
Visible crew/equipment: As Arch Hammer is walking to Hotel Real, the camera is looking up towards him as he walks. Just before he turns to go in, the lights, roof, and support beams of the soundstage are above the skyline backdrop.
Visible crew/equipment: Right after the phone conversation, everybody starts going towards the door. As they do, if you look at the couch on left you can very briefly see a shadow of something moving across it. It couldn't be any of the actors since they were all either outside or on the other side of couch.
A Thing About Machines - S2-E4
Visible crew/equipment: When Finchley is being chased by his "driverless" car, you can see the driver crouching down at the wheel.
Visible crew/equipment: An equipment shadow moves across the back of Ellington's coat as Brother Jerome is telling him how the Devil was captured. (00:14:30)
The Lateness of the Hour - S2-E8
Visible crew/equipment: As Robert is pouring Dr. Loren's drink, an equipment shadow moves across the top of Jana's head in the foreground. (00:08:20)
The Lateness of the Hour - S2-E8
Visible crew/equipment: The boom shadow intrudes briefly at the upper left of the shot as Jana stands at the window and says, "Rest in peace." (00:16:15)
The Lateness of the Hour - S2-E8
Visible crew/equipment: When Jana is screaming "No pain!" at the top of the stairs, a film crew member's shadow can be seen moving on the wall behind her. (00:22:30)
Visible crew/equipment: When Sykes opens the Sheriff's office door to leave, the "sky" outside has a very large equipment shadow on it, hanging over the building across the street. (00:05:10)
Mr. Dingle, the Strong - S2-E19
Visible crew/equipment: In the scene where Dingle is waking up to the alarm clock, you can see a camera shadow moving over the bed.
Visible crew/equipment: When Chris escorts Grandma Bayles up the stairs, a blocking mark is plainly visible at the lower right, chalked on the carpet. (00:05:25)
A Hundred Yards over the Rim - S2-E23
Visible crew/equipment: When Christian walks up to the man in front of the diner, you can see a crew member in a plaid shirt and white pants reflected in the man's sunglasses.
The Mind and the Matter - S2-E27
Visible crew/equipment: While Beechcroft talks to his boss in the men's room, the boom shadow moves across the wall at the upper right of the screen. (00:03:35)
The Mirror - S3-E6
Visible crew/equipment: Clemente walks over to the portrait of the former ruler, and throws his glass. Just as he completes the action, the camera looks at the portrait. There is a movement of what appears to be a device on the right side of the shot firing the glass.
Hocus-Pocus and Frisby - S3-E30
Visible crew/equipment: When Frisby tells the two travellers that they're looking at the grandad of the American automobile, we can see the reflection of a crewmember wearing a suit, possibly the director, who stands up from a directors chair (also seen in a following shot). Then as Frisby tries to locate the gas tank, the shadow of the moving boom mic is visible on the store's doors. (00:06:20)
Hocus-Pocus and Frisby - S3-E30
Visible crew/equipment: After the two travellers have paid for their gas, when Frisby advises them to check in someplace because of the clouds, the reflection of the moving boom pole and mic can be seen above the word "shop" on the store window. (00:08:00)
Death Ship - S4-E6
Visible crew/equipment: Just after the ship re-attains orbit, the shadow of a production crew member moving his arm can be seen on the wall at the upper right of the shot. (00:38:30)
Death Ship - S4-E6
Visible crew/equipment: In the final shot of the ship ascending into space, one of the filament lines supporting the model is showing just above the saucer. (00:44:15)
On Thursday We Leave For Home - S4-E16
Visible crew/equipment: As the men talk in the cave, the boom mike is plainly visible.
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.