Revealing mistake: During the Yule Ball there is a shot of Professor Moody sitting on a bench tapping time with the music. One can easily see a green object beneath the bench. It is apparent that a hole was cut in the cushion with his real leg hanging below the bench covered with a green leg and foot sleeve. The large green object is his foot with a red dot on his toe. In post production the actor's real leg and foot is to be removed digitally via green screen-like technology and replaced with a digitized artificial one. The red dots are to help coordinate the movements and position of the artificial leg with the actor's real one. This shot obviously got missed by the postproduction team. If you look closely you can also just make out his knee peeking below the fringe of the bench. (01:18:50)
Continuity mistake: When Harry returns from the cemetery, there's a shot of him lying over Cedric's body. When Dumbledore reaches out to put his hands on Harry's face, Harry's left shoulder is higher while he rests his weight on his right arm. The scene cuts to a rear view, and his right shoulder is now higher. When it cuts back to a front view, his left shoulder is once again in the higher position.
Continuity mistake: In the scene where Hermione makes Harry stand up after his name has been retrieved from the Goblet of Fire, Ron continues to look straight ahead and has an emotionless expression for that shot. In the next shot when Harry is walking to Dumbledore, Ron's head is suddenly slightly down and moving up towards Harry, and a glare has appeared on his face.
Continuity mistake: When Cedric puts his name into the Goblet, you can see that his hair is wet on his fringe and stuck to his forehead in several clumps of hair. After he has stepped away from the Goblet, his hair has changed position. The clumps of hair are in different places and a gap has appeared. (00:28:00)
Continuity mistake: During the first challenge, the arena/stadium is significantly smaller during the bird's eye view CGI scenes than the physical scene's interior shots.