Revealing mistake: In the Forbidden Forest, HRH are introduced to Grawp. Just as the giant lifts Hermione off the ground, in the shot from behind her, Hermione's legs disappear below her calves; the digital lines are actually visible. Slow-mo is not necessary. (Only visible on fullscreen DVD.)
Revealing mistake: In the Department of Mysteries, when Lucius removes his mask Bellatrix stands behind him, but her entire body has been digitally blackened. Additionally, there are two distinct shots in which there is a digital shape highlighting Bellatrix's face/upper body that moves as she moves. The first occurs when she remarks, "He knows how to play," then after Lucius begins to say, "Prophecies can only be retrieved," when he continues with the words, "by those about whom" it occurs once again in the close-up.
Revealing mistake: When Harry is going down to Hagrid's hut and discovers he isn't home, he looks over the forest discovering the thestrals. The camera moves back so Harry is also visible in the shot, but as he becomes visible, just shortly at his head to the left some of his head goes missing showing that it is a chroma-keyed background / blue or green screen.