Lethal Weapon 4 (1998)

Deliberate "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 CLEARLY see that it is not Mel Gibson, but an Asian stunt man.

Continuity: 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.

Continuity: At the end of the movie when Riggs, Murtaugh and the other two cops enter the garage where the Triad chief is buying the four fathers' freedom whe can clearly hear the hard rain outside, but the car that they're driving is completely dry - maybe they had a very big umbrella!

After Riggs shoots the valve on the napalm tank being worn by the criminal with the flamethrower at the start of the film, the legendary "Wilhelm" Scream can be heard as the criminal goes flying.

Jackie Chan was originally offered the role of Wah Sing Su, but turned it down because he chooses never to play a villain in a movie.

In one of the takes when Riggs and Murtaugh crashed through the window of the office building, one of the window washers was actually hit (from the "making of's"). Obviously the one in the movie was not this take.