Revealing mistake: In the wizards versus werewolves special, it's painfully obvious Juliet is a stunt double when she fights and grows her wings.
Revealing mistake: After Sam shoots Spencer with her blowgun, she lands on the ground after jumping from the ceiling. During this shot, you can easily see that it is a stunt double.
Goodbye, Sarah Jane Smith: Part 2 - S4-E12
Revealing mistake: In the shot in Sarah Jane's attic right after Ruby is thrown into the sun, the group are talking in the attic and to the left of the screen, you can see that the blue screen used to create Mr Smith is visible.
Race to the Spaceship/Sandbox Surprise - S3-E5
Revealing mistake: In the sequence where Pig is making a pizza with strange toppings, an ant caries a large bucket of popcorn up the counter. It is flush on its back with no top when on the floor, and the popcorn is bulging out. The ant turns 90 degrees and walks up the counter, and the popcorn thus turns on its side in the air. Not one piece of popcorn falls out or even shifts, despite the pile now pointing out rather than up.