Other mistake: In the scene where Ripley throws the basketball at Johner, Christie says to Ripley, "Ok, I've got a new game, tag .." He hits her with the handles straight in the face once, he misses the next shot, misses again, and Ripley hits him to the ground, but you neither see or hear the handles fall. (00:23:15)
Other mistake: On his own computer, Sean looks up his son's file, which states the boy died immediately and his date of death was September 9th, 1991. When Eve and Castor (as Sean Archer) are at the cemetery, the date of death on the headstone is September 24th, 1991. (00:21:15 - 01:14:50)
Other mistake: In the scene in the beginning when you first see the fifth element, and the camera moves up so you see the wide open mouth, look to the shoulder on the left. In the years of undisturbed dust, there are a few finger prints. (00:09:35)
Other mistake: Toward the beginning, when the characters wonder aloud what the rumbling noise is, someone suggests that it's the ventilation system, to which Rennes responds "No vents." However, in the passages between rooms there are panels with ventilation slits in them (which you can plainly see even as Rennes says the line), and it's pretty impossible that the characteristically level-headed and observant Rennes wouldn't have noticed that.
Other mistake: Just after Rico and Ace rescue Carmen, the other guy has hold of the nuke, which explodes. Watch when the rocks collapse behind the three characters, you can see a nice square block lower from the ceiling, almost like a door being lowered. The crew could perhaps have made it look a little more realistic. (01:55:00)
Other mistake: In the final scene outside the MIB headquarters, Kay opens the neuralizer and we see it open, but there is no sound. Later when Kay tells Jay that he hasn't been training a partner but a replacement, we hear the sound of the neuralizer opening and Kay tells Jay how to operate it. (01:27:45)
Other mistake: During the Cyborg Master's attack on the police station, the bald cop goes to get a gun out of the gun storage room - which is a regular closet. It's not even locked, he just walks straight in. No police station in the world would be that careless with their weapons.
Other mistake: Near the end of the film a helicopter crashes into the jungle. For one, the greenscreen is EXTREMELY obvious, and secondly, the helicopter is clearly a toy, and even has writing on it (like 'Made in Taiwan').
Other mistake: When Abby and Costner is on the run and hide in the forest. They kill a Holnist and capture his horse. When leaving, they still only have one horse.
Other mistake: When they have the baby Rex and the parents approach for the infant, there's an outside shot of the T-Rex roaring as Ian moves away to the other side. After he does that the Rex on the other side silently roars, then that same roar bit is repeated from inside the trailer in the very next shot, but the roar is added in, showing repeated takes.