Revealing mistake: It is not what is seen here that is the problem, but what we don't see on the Hogwarts Express; first on the way to Hogwarts when Harry, Ron, and Hermione are in their compartment talking about Draco, and second, during holiday break, when Harry and Ron are talking about Unbreakable Vows. As the camera faces the large compartment window, we see the scenery out the window, but as the train speeds along, the scenic reflection of the windows opposite it, in the corridor, should be seen but isn't. What is reflected is the light colored wall of the compartment beneath the blacked-out window. The camera filming the shots from this angle is positioned at that window, and to hide the camera/crews' outline, the window was covered.
Revealing mistake: Immediately after Katie is cursed by the necklace, Harry, Ron and Hermione are in Mcgonagall's office talking about what they saw in Hogsmeade. Harry says, "It was Malfoy." As both teachers turn Mcgonagall says,"That is a very serious accusation, Potter." As she says this, she looks oddly transparent. You can see the window pane design through her robes, particularly in the shoulder. This continues to be true until she says "I suggest you go back to your dormitories." This could possibly signify a green or blue screen behind them. (00:54:10 - 00:54:40)
Revealing mistake: Harry and Dumbledore apparate onto Professor Slughorn's street, and Harry says "I just apparated, didn't I?" As Dumbledore is answering, ("Indeed. Quite successfully too, I might add.") the shot pans out. Look at Dumbledore's "burnt" hand. The green glove they used to make his hand look blackened is visible on his fingers. It is also visible when he turns and starts walking away from the camera. It is not due to the lighting, as it is not visible in the next shot as they both walk away. (00:05:15)