Other mistake: When Dylan is transported from the stage to "the vault", Alma, Thaddeus and Thaddeus' assistant manage to get down the stairs to "the vault" just a few seconds after he lands as if they took a simple walk there. How would they get there that fast anyway? Dylan got to "the vault" by going through a "teleportation device" that went straight to the fake vault under the stage, but the other three had to walk down a flight of stairs to get there. Even with the fake vault being close to the stage it still would take longer to get down to the fake vault. To begin with, when the vault opened, the stairs they walked down were visible several meters in the background. There were about 13 steps on the staircase which would have made the width of the staircase about 3 meters and the height about 4 meters. With the staircase at this length and position, the staircase leading to the fake vault would have had to have been located around the sitting area. Alma, Thaddeus and Thaddeus' assistant managed to get to the fake vault in about 6 seconds, too quickly. (00:36:05)
Other mistake: When Henley is supposedly getting eaten by the piranhas in the tank, loads of bloody water keeps spilling out of the tank and even lands on a guy in a blue hat hitting the tank. As the shot slowly pans over the mysterious man in a hoodie, the guy in the blue hat is somehow completely dry. (00:07:10)
Other mistake: When the Horsemen remove $70,000 from Tressler's cheque, audience member Josepha is waving her flashlight on her paper, and begins to notice something. However, it is not until the next shot where it is shown that the 70 has appeared in front of her $562 written on her paper. (01:00:50)
Answer: The mirror was inserted at an angle, so that the reflection was the ceiling and not what was in front of them. It's the same principle for how periscopes work.
Bishop73