Continuity mistake: When the bottle falls on the shop floor, it changes positions between the time it settles and the moment it begins to smoke. (00:05:50)
Revealing mistake: Mr. & Mrs. Castle wish for a million dollars in 5 and 10 dollar bills. But the bills they receive all have the same pattern printed on both sides, a pattern which which doesn't resemble any real US currency. There's also not nearly enough of the bills to total a million dollars. (00:12:30)
Factual error: The Nazi symbol on the flag is reversed.
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.