Factual error: Most of the action is set in 1988. The prologue with the two little girls is set before or during that year, but the girl scouts are selling "Thindulgent Sandwich cookies", by Sheila G, a brand established after 1992. (00:01:55)
Revealing mistake: Obvious use of green screen for buildings across the street. On and off, the background fades, and buildings appear higher or lower depending on the camera's angle. At one point, the angle of walls behind Rex (Bruce Willis) is at a downward angle, and his head is almost touching the ceiling. (00:06:10 - 00:07:25)
Plot hole: SPOILER ALERT! During the final fight, Joey kills Frank and she reverts back to human form. Abigail tells her she's fine, because Frank was the one who turned her. However, when Frank kills Lambert he does not revert back to human form, even though Lambert was the one who turned him.
Suggested correction: Because it was too late for Frank, who had already turned, Joey hadn't.
If that were the case, then why would Abigail bother explaining to Joey why she reverted back to human form?
Continuity mistake: When Alicia puts flowers on her mother's tomb, she casts a shadow on the gravestone in side view, but none with the camera behind her back. (00:03:05)
Factual error: Towards the end of the movie when Superman and Henshaw are fighting on the Watch Tower in space, Lois hits the sun shades to lower them so that direct sunlight can hit Superman and power him up. This is out in space with no atmosphere to filter the sunlight. The reason the blockers were there was for this reason, to keep the harmful unfiltered sun light from hitting people on the ship through the windows. However, she is able to stand there in the direct light after it's done and kiss and hold Superman without being hurt or burned by the intense direct unfiltered sunlight or its radiation.
Continuity mistake: When the guy leaves the motel, he only grabs his jacket, but when he's outside, he has a helmet in his hand. (01:14:55)
Visible crew/equipment: As the camera moves forward in the opening shot, a clear reflection of a film light can be seen on the side of the fire engine.
Other mistake: The opening (like most of the movie) does not make much sense, so you can pick your kind of mistake here; the special effects are cartoonishly bad throughout the big firework explosions. When John Abraham is knocked through a window by a rocket (which takes off with a 270° turn without even rising) to the face, he gets up unscathed and starts screaming for Junky and Chandu. The camera moves from them to him and he's running with the girl in the red costume in front of him (which was way behind, safely, when he crashed through the window, so already that is wrong), and then there's a wider shot. In that shot, you can tell that Adams is bumping into the same people as before and the distances between the extras are all wrong. (00:06:35)
Revealing mistake: When "Bruce" was standing above the man ("Not Mr. X") and pressed her index finger against his forehead to push him down the steps, the man's feet were not even touching the steps. (00:21:22)
Character mistake: Casey is sixth of six male students standing in a row to individually demonstrate a karate move for Sensei. After the fourth student, Sensei passes up student #5 and says, "Next. Casey." Sensei completely missed the fifth student, the only other beginning white belt like Casey. (00:28:45 - 00:29:10)