Hocus-Pocus and Frisby - S3-E30
Revealing mistake: Make-up for the space aliens was apparently changed after the closing scene in the store was shot and before any footage of the aliens' "true appearance" was actually taken. This resulted in Frisby, who is now telling the truth and finding no one to believe him, saying that the aliens had a single eye in their foreheads, when in fact, the aliens each had two eyes, in the traditional location. (00:24:00)
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.