Audio problem: When Markie is finally revealing to Helen who she is, Terry Burnham (Markie) is standing behind Janice Rule (Helen) talking to her over her shoulder and can be seen mouthing Janice's lines along with her in between her own.
Other mistake: Helen is holding the cup of rich "cocoa" that Markie supposedly just drank. Helen says, "I thought she'd finished her cocoa, and here it is untouched." Close-up on the "cocoa," which is clearly (so to speak) nothing more than water. (00:12:10)
Revealing mistake: When "Selden" falls down the stairs it is obvious that it's a stunt double.
Continuity mistake: When Helen pushes Seldon to his death, his body was facing to the right, but in next shot he is facing to the left.
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.