The Tale of the Pinball Wizard - S1-E13
Visible crew/equipment: When Russ is in the royal chair and it is moving on its own, right after it hits the witch you can see a crewman behind the chair pushing it.
The Tale of the Whispering Walls - S2-E7
Factual error: Betty Ann is supposed to be telling this story on a Leap Year night with a full moon. While it's necessary for the plot, the most recent Leap Year with a full moon was in 1972, and is calculated to happen in 2048 next.
The Tale of the Mystical Mirror - S5-E3
Plot hole: It makes no sense that Ms. Valenti gives all of her employees a compact mirror and also invites them to her home. Any of them could open their mirror while standing near a painting of Ms. Valenti and they would discover the secret, as Cindy does.
Suggested correction: The compact mirror includes a palette of cosmetics. Cindy shows it to Laurel, who says, "She gave one to me, too, like we're ever gonna use it." Presumably, the employees are mostly using the free samples of cosmetics from the shop displays. I myself have owned some compacts that I didn't always want in my purse (if they were too big or awkward in a certain purse).
The Tale of the Manaha - S5-E7
Character mistake: Near the beginning as the leader is about to set off with the boys, he says, "About face!" and turns to the left, and the boys follow suit. But "About face" means to turn 180°. He should have said "Left face" to get them to turn 90° to the left. (00:04:25)