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
Revealing mistake: During the big gun battle right before the last fight scene, Riggs is shown driving a forklift, and knocking over some barrels on top of Wah Sing Ku, leader of the Triads. When the forklift does the 180° turn, you can see that it is not Mel Gibson, but an Asian stunt man.
Leo Getz: They fuck you with cell phones. That's what it is. They're fuckin' you with the cell phone. They love it when you get cut off. Y'know why, huh? You know why? 'Cause when you call back, which they know you're gonna do, they charge you for that fuckin' first minute again at that high rate.
Question: In the beginning of the movie when the tanker truck gets blown up, why does the fully intact truck just get launched straight up with flames below it? The source of the explosion would be the truck itself, so when ignited, the truck should have simply been blown up into a million pieces, not launched straight up fully intact.
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Answer: This was most likely done just to make the scene look that much cooler. Movies do things like this all the time.
Carl Missouri