Revealing mistake: At Jareth's castle, in the room with the Escher stairs, Sarah sees Toby and she jumps. It's very apparent that it's a stuntwoman jumping from the ledge in the wide shot. (01:30:20)
Revealing mistake: When Sarah discovers Toby has gone from his cot, Goblins start moving about in her fathers room. The second goblin we see laughs, and jumps down from the top of her fathers four poster bed. As the goblin does this, you can see the shadow of a boom mic bob down onto the material that drapes from each poster of the bed. (00:11:05)
Revealing mistake: Sarah looks at Hoggle in her vanity mirror and he says, "Well why didn't you say so?" As Sarah starts to stand up it is obvious where the two shots are edited together with her back to the camera, so that at the end of this "one" shot, Sarah's room is filled with her new friends. Her vest's back ties and blouse sleeves are different in the two shots that are edited together as one. (01:35:35)
Revealing mistake: Sarah walks over to Ludo as he hangs upside down. Just before she says, "Now, stop that," she takes a step back as Ludo growls and shakes his head around. On his upside down scalp inside his fur a metal clamp on the black material under the fur is visible. (00:43:05)
Revealing mistake: Back in her room Sarah puts things away in her vanity drawer, including her music box. The diameter of the music box is greater than the height of the sides of the drawer, which would prevent the drawer from closing properly. On the white front of the vanity table above that drawer, the top lip is cut away to allow for the important shot of Sarah closing the drawer. (00:34:40)
Revealing mistake: During the 'Dance Magic Dance'-sequence in Jareth's throne room, David Bowie throws up a baby dummy (it's near the end of the song). (00:26:00)
Revealing mistake: After climbing up the ladder, Hoggle stands up in the large pot, then Sarah climbs out in the foreground and Hoggle is seen raising his body, to climb out of the pot too. However as Sarah lifts her leg out of the pot, Hoggle is seen lowering his body back into the pot. In the next shot he is beside the pot as if he just climbed out. (00:38:00)
Revealing mistake: Numerous times you can see the 'metal' robot's metal armour and the spikes on the armour wobble like rubber. This is because the whole robot's armour is made of polyurethane foam. (01:13:30 - 01:15:00)
Revealing mistake: In many shots throughout the film it is very obvious when it is a blue screen, plate and/or matte painting shot, by the area around the subjects and/or the subjects' appearance or disappearance onscreen. Some examples can be found with the Fireys and Sarah, during the Chilly Downs scene, and later, when Sarah breaks the glass with the chair, part of her face disappears when she stands with the glass behind her. (00:51:00)
Revealing mistake: Jareth sends a crystal ball to turn into the cleaners in the Oubliette. Right after the cleaners appear, the camera turns to Hoggle and Sarah, where Hoggle says, "Oh no, the Cleaners." Soon after, the two of them start to run. As they start to turn, look above at the ceiling. A light turns on to show them the way out.
Revealing mistake: When Sarah is trying to reach Toby while in the Escher-style room, there is a scene where Jareth is walking along (underneath where Sarah is standing), he does an 180-degree flip around the edge of the platform. If you look at Jareth's shadow, you can clearly see the shadow of a support bar pushing him up. (01:24:45)
Revealing mistake: When in the oubliette, Hoggle lights one candle, and it's flame seems to be able to light up the whole of the oubliette very brightly, much like studio lighting. (00:21:25)
Revealing mistake: It is obvious when the owl, that attacks Sarah, is a puppet and when it is a real owl, in consecutive shots. It is also obvious when the snake, that Jareth tosses at Sarah, is real and is a fake in subsequent shots. It is also very apparent when Sir Didymus' dog, Ambrosius, is a real Sheep Dog, or is a puppet Sheep Dog, in consecutive shots as well. (00:11:50 - 01:21:25)
Revealing mistake: Just after they have all entered the 'Goblin City' and the giant robot materialises from the two gates, Sir Didymus's dog, Ambrosius (the real one - not the puppet) bolts and runs off towards the "metal" spikes. As the dog gets to the spikes they move and wobble - unlike metal would. (01:16:20)
Revealing mistake: When the girl starts walking along the labyrinth check a V shaped tree on the right. She's always walking that very same track revealing the set was small and perspective was achieved with matte paintings.
Revealing mistake: When Sarah takes the chair to break Jareth's glass bubble, the flower adorned lace topped table is seen. Then as she breaks the glass the table is visible once again, however, the way the tablecloth lies and the flowers, cups, etc., are all positioned differently. (01:09:10)
Revealing mistake: When David Bowie throws a crystal ball into the corridor it is the start of the cleaner scene. The frontdrill of the cleaner appears in the distance. This is animated as it rotates fast. Once the actual device is visible the drill rotates in fact fairly slowly. (00:34:25)
Revealing mistake: In the Goblin City, toward the end of the fight scene, one big boulder rolls towards a group set up like ten pins. As it hits them, you can see the plungers used to pop the dummies in the air.
Answer: To add to that, the other door was supposed to lead her to certain death, and technically the oubliette wasn't exactly certain death.