Black Leather Jackets - S5-E18
Revealing mistake: The aliens cause a power fluctuation, and Mr. Tillman complains that all the lights in his house are flickering on and off. But the lamps in his living room are burning steadily. The "flickering" light is originating from what is obviously stage lighting above the set. (00:05:15)
Revealing mistake: The strings on the guitar are extremely loose. While they would make a sound, it wouldn't be anywhere near the proper sound. The guitar sound throughout the episode doesn't match up to the strokes of the guitar either.
Nightmare at 20,000 Feet - S5-E3
Revealing mistake: At one point, when the gremlin is about to rip open part of the engine, you can see he has a white sole on his foot, part of the costume.
Mr. Garrity and the Graves - S5-E32
Revealing mistake: When the town's people are waiting for Garrity's arrival at midnight, we hear an old clock informing everybody it is midnight and they all turn around to see. But look at the time on the clock - you can quite clearly see that it is actually five minutes till 6 o'clock.
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.