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.
Revealing mistake: The shark Leo catches is unbelievably fake - you can even see the seams.
Revealing mistake: When Riggs jumps from the car onto the house that's being moved, you can see when the stuntman replaces Mel Gibson. Look at his hair, it changes styles as he jumps.
Revealing mistake: During the car chase when the Pontiac is about to jump into the office building, you can see a white stunt driver from the left side mirror, though Murtaugh is driving.
Revealing mistake: In the scene where Riggs and Murtaugh's vehicle breaks through the glass to get back on highway, the vehicle sparks even before it hits the pavement.
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.