Continuity mistake: After mowing down all the Immortals during the Sanctuary scene, one Watcher tells another to take their heads because they might be Immortals. You see the Watcher to the left side of the screen and a robed figure to his left about four feet away drawing his sword. Suddenly, that same robed figure is now to the Watcher right side and about two feet away.
Continuity mistake: During the final duel, Kell traps Duncan's sword against a guardrail and then kicks it into the air. Duncan jumps off and grabs the sword with his left hand. After he hits the ground, the sword is suddenly in his right hand.
Deliberate mistake: In the Director's Cut (the version most readily available on DVD), there is literally an entire 15-second chunk of the final fight that is repeated twice within 60 seconds. (It's the bit where Duncan and Kell are fighting on the ground, Duncan kicks Kell, then Kell kicks and slices Duncan) It's impossible not to notice, and it's not they're flashing back to what just happened... they just repeat the same 15 seconds of footage, add different sound effects, and assume people won't notice.
Continuity mistake: While Duncan is riding in the taxi, the numbers on the side change from shot to shot.
Other mistake: In the scene where Duncan has been captured by the Watchers and restrained in the upright table, take note of the needle. The camera pans around the Watcher's head and then shows the syringe delivering its contents. Once the dose is fully administered, it can clearly be seen that there is no needle on the end when he holds it down near the Highlander's chest.
Visible crew/equipment: There is a shot where the taxi is on the bridge going into NYC, and when the camera that is tracking the taxi pans, we catch the corner of the truck that the camera is mounted on.
Continuity mistake: When Kell is stabbed by Connor at the beginning of the movie, the sword is sticking out from his back about one and a half to two feet. Yet when he lands the sword doesn't seem to matter since he is laying flush on his back.
Continuity mistake: After Joe shoots and kills Hale, it cuts to a wideshot of Duncan and Kell on a catwalk squaring up against each other. Duncan makes a very specific move, where he glides his sword from his right side to his left. The shot then cuts to a medium shot... and Duncan's sword is suddenly on his right side again, and he makes the exact same move a second time.
Continuity mistake: During the flashback to 1712, Connor and Duncan fight it out with highwaymen who are collecting fake tolls from travelers. Throughout this scene, Duncan McLeod's five-o-clock shadow starts out dark, becoming lighter, then darker, and then almost clean-shaven from shot-to-shot as the fight progresses. This continuity problem persists, to a lesser extent, throughout the film.