Audio problem: Right as Duncan leaps into the "chain room" in the final fight, he lets out a light "Ugh!" sound. Then, about 5-10 seconds later, when Duncan surprises Kell and kicks him... Kell makes the EXACT same "Ugh!" sound effect.
Highlander: Endgame (2000)
1 audio problem - chronological order
Directed by: Douglas Aarniokoski
Starring: Christopher Lambert, Adrian Paul, Bruce Payne, Lisa Barbuscia
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.
Connor MacLeod: Life brings hope and pain, but revenge never brings redemption.
Trivia: The character of Manny, one of Kell's posse, is played by Vernon Rieta, who has been Adrian Paul's Hung Gar Kung Fu teacher for years.
Answer: It seems some catastrophic event occurs. This movie follows the continuity of the television show. In one episode Joe Dawson tells Duncan MacLeod the last time it happened was when two immortals fought in a temple at the base of a volcano. He tells him this caused the eruption that consumed Pompeii. But, Dawson also specifies that it's only a legend, and can't be confirmed as actually being THE specific reason for Vesuvius's eruption.