Deliberate mistake: When Riggs is chasing the guy on foot, you can clearly see it is a stunt double, who is also very apparent in other shots throughout the film.
Lethal Weapon 4 (1998)
1 deliberate mistake - chronological order
Directed by: Richard Donner
Starring: Mel Gibson, Danny Glover, Chris Rock, Jet Li, Rene Russo, Joe Pesci, Kim Chan, Steve Kahan
Continuity mistake: During the car chase involving the car driven by the Chinese guy, Murtaugh, and the mobile home, the two cars slide against each other and the right wing mirror on Murtaugh's car is completely wrecked. Once the two cars are separated the wing mirror has miraculously repaired itself. The left wing mirror also does a disappearing / reappearing trick.
Roger Murtaugh: You ever hear of Ebony Clarke?
Martin Riggs: Yeah, she writes those cheesy sex novels. Why? You boinkin' her?
Roger Murtaugh: No I'm not boinkin' her! Trish is Ebony Clarke!
Martin Riggs: Oh, so you ARE boinkin' her.
Roger Murtaugh: Yeah... Yeah, I'm boinkin' her!
More trivia for Lethal Weapon 4
Question: I wonder, what was the point of having a gunfight with the armored man at the beginning of the film?
Answer: It was put in the film to introduce the concept of Rianne and Lorna being pregnant and to bring in the joke about the news article about Roger, not to mention giving the film an action-heavy opening.