Continuity mistake: In one scene, Liv Tyler's character informs another character that they suffer from halitosis. The film is set in the 1700's, and the condition of halitosis was invented in the 1930's as a method of selling fragrances and deoderants. Therefore, the condition she refers to doesn't even exist at that time.
Revealing mistake: When the Interceptor team flies in their cargo plane to Mexico to investigate the UFO crash, several shots filmed from the front show the six of them all in the cockpit (not in the body of the plane.) They're seated as they might be in a car, three across, front and back. The window of the cockpit looks remarkably similar to a station wagon windshield, down to the large rectangular front windshield, and even the rear view mirror is in place.
Continuity mistake: Just as the final fight between Luc and Seth is about to begin, three computer panels are shown to display Seth's different approaches to crack the code he requires to survive. While the two LCDs to the left and right have the output of a brute-force algorithm simply guessing all possible combinations for the code, the CRT-monitor in the middle shows Luc's severely injured daughter which Seth offered to heal in exchange for Luc telling him the code. However, if you have a closer look at the screen it states "HEALING COMPLETE" (even with a blinking frame around it), although close-ups of the "Rejuvo-booth program" before (e.g. at 1:08:00) and after (e.g. at 1:09:24) this shot claim it to be "INCOMPLETE". (01:08:55)
Revealing mistake: When Jamie Lee Curtis falls into the fuel tanks, how could she be breathing? The gases would be too toxic for her to breath, and wouldn't she be blinded by the fuel?
Other mistake: At the start of the film where Detective Dooley and Jerry-Lee are getting ready for work, we see Dooley climbing out of the shower, but in the reflection in the mirror behind him, we see him emerging and can clearly see his under-pants which are bone-dry. (00:00:32)
Other mistake: When the one-legged judge bends over to retrieve his wooden leg from Paul Newman's grandson, you can see the judge's real leg and foot sticking out behind him.
Continuity mistake: When Beowulf is rescuing Pendra, at the end of the fight he is holding is sword over one of the men. In some shots it is clean, in others it has blood on it. (00:08:00)
Revealing mistake: In several scenes (e.g. in the deli & in the tunnel, at the end) the ice cubes used to cool Elvis down are obviously not real ice.
Visible crew/equipment: When the firetruck arrives you can see the entire crew reflected in the side of the truck.
Continuity mistake: In the scene when Nick Chen (Chow Yun-Fat) talks to Jack (Jon Kit Lee) at the NYC Correction, the shot facing Nick shows him grabing Jack's face by the chin. The next shot, facing Jack, shows Nick's hand around Jack's neck. The final shot facing Nick again shows his hand back on the chin. (00:58:50)
Audio problem: When Alex is appointed head of Netforce, he looks down for a moment and says, "I don't know what to say." And then looks back up. His mouth is in clear view the full shot, yet never moves once during this line and is completely shut. The audio was just dubbed over the footage in post. (00:23:25)
Other mistake: Lieutenant Fredricks introduces himself to Agents Riley and Baker as "Lieutenant Fredricks," and that's the spelling on his uniform's lapel. However, he is listed as "Lt. Fredericks" under the cast. (00:57:37 - 01:27:07)
Continuity mistake: As Admiral Wilson on the Pegasus base fires his weapon on the glass window that protects the Navigation A.I. it doesn't gets a scratch. In the next shot he starts to hit the glass with the rifle's stock and a large grey stain have appeared on the glass, only to have vanished next time the glass is shown.