When the Cutie Mark Crusaders start fighting, Twist (one of the students) doesn't have her Cutie Mark.
In the three diamond reflections that Rarity runs into, Discord's fang is on the right of his top lip the first time we see him in the reflection, but in the next shot when we see him, it's on the opposite side.
During the close-up shot of Celestia telling the Mane Six to use the Elements of Harmony to take on Discord, her tail is not waving.