Continuity mistake: During the funeral pyre scene, the folds in Obi-Wan's hood change between consecutive shots.
Continuity mistake: When Palpatine and Queen Amidala address the Senate, the holocam flying around near them is inconsistently placed between shots.
Continuity mistake: When R2-D2 and three other astromech droids fixing the Nubeing ship, Jar-Jar is talking to them and there is a black one. This black one turns into a blue one when they are fixing the ship. It's the second one to get blasted.
Continuity mistake: During the climatic light-saber duel, just as Qui-Gonn and Obi-Wan have forced Darth Maul onto a ledge, look quickly and you will see that the red blade of Maul's light saber goes from being behind his waist to in front of it between shots.
Continuity mistake: At the beginning of the Battle of Naboo, the Gungans create a shield around themselves using a number of shield generators on the backs of their mounted creatures. In a later scene, only one of these creatures has a shield generator on its back.
Continuity mistake: After Obi Wan halves Darth Maul, his lightsaber is in his hand as his body falls downward. Next shot is overhead and no saber is in hand or plummeting down the shaft.
Continuity mistake: The shadows change angles all over the movie. First the shadows appear right beneath the characters, then in the next clip they are cast with an angle. And the shadows that ships and computer-animated animals cast are different from the "real" characters.
Continuity mistake: When Anakin says his last words to his mother, she is closer to the door than in the previous shot.
Continuity mistake: Tey How says that she lost the transmission. The screen behind Nute Gunray is blank. A few shots later, the transmission works again, although no-one went to the bridge corridor and fixed it.
Continuity mistake: At the start of the lightsaber fight in the hanger all three go through some large doors and into the big generator room. Behind them a set of stairs have appeared.
Continuity mistake: When Obi-Wan runs towards Darth Maul and Qui-Gon he is halfway down the walkway, but when it cuts to Darth Maul and Qui-Gon Obi-Wan is now at the far end.
Continuity mistake: When Obi-Wan is hanging from the ledge in the abyss after Darth Maul knocks him into it, he sees Qui-Gon's lightsaber a distance from him and uses the force to grab it. Between shots, the lightsaber changes position on the floor.
Chosen answer: According to "Inside the Worlds of Star Wars: Episode I" by Kristin Lund, they are "laser doors which lock into position in response to potentially lethal power outputs that occur intermittently during plasma activation process" (the fight takes place in a plasma energy processing plant). There are six laser doors in deference to an ancient Naboo legend in which Chaos is held back by six inpentrable gates. As you say though, they're really just a plot device to add dramatic tension to the duel!
Sierra1 ★