In His Image - S4-E1
Revealing mistake: In the basement lab scene at the end, George Grizzard's double's face is visible in profile several times. (00:38:50)
Revealing mistake: We see Redfield's car strike the invisible barrier from a side angle. It jerks to a stop and the hood flies up, but there's no other front end damage - until the angle shifts to a forward view, a shot of an identical car that had been previously wrecked. Now there's suddenly considerable damage to the front end, and the hood is in a different, much more crumpled position. (00:11:10)
Death Ship - S4-E6
Revealing mistake: When Mason puts the ID card back into the pocket of his dead duplicate, the "corpse" twitches its eyelids. (00:14:30)
Of Late I Think of Cliffordville - S4-E14
Revealing mistake: When the camera is in a close up on Feathersmith's face, you can see the line of the skin wig he wore to be older.
Of Late I Think of Cliffordville - S4-E14
Revealing mistake: When the camera is in close up on Feathersmith, the line of the skin wig is clear.
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.