Revealing mistake: In the office tower after the quake there is a panic and workers rush down the stairs only to find them fallen away. After two women fall to their death a man is left hanging on a girder, but falls onto a giant piece of glass. Look at the top of the frame and you will see the airbag shooting our from under it.
Revealing mistake: When the airplane is coming in for a landing, we see an exterior shot of the plane shaking violently well before it has touched down. Clearly the "earthquake" effect is achieved by merely shaking the camera.
Other mistake: During the quake, we see the furniture, wall panels, ceiling tile, windowpanes, vertical blinds, etc., in the office fall, shake, or crack. Yet the leaves of a large potted plant do not move at all.
Continuity mistake: During the scene at the airport when the tower OK's the plane to land we can see it is a 707 with TWA-like colors of red and white. Then when there is the shot of the plane on the ground it turns into a DC-8 with orange markings. When the plane takes off into the air the camera shows the 707 again.
Revealing mistake: During the quake, a mother is seen having a plate glass window fall on her face while her daughter screams. Before the mother turns around to show us the gore, look close and you can see a shard of glass already "placed" in her face before the fake glass falls her. Additionally, while not a "mistake", it's clearly a "stuntman" portraying the mother. The face and yelping give it away.
Continuity mistake: When the aftershock hits L.A. and destroys Wilshire Plaza, the camera shows the destruction underground and at some point a brick pillar falls over. Instead of being made of brick it's really made out of wood with brick type wallpaper. You can tell this because if the pillar was really made of brick, when it fell over chunks of brick would at least break off. (01:38:20)
Continuity mistake: At the beginning of the earthquake, outside of the office building the entire scene is out of continuity. Just watch the woman with the orange dress.
Revealing mistake: After failing to spin the loop in the motorcycle the first time, Miles tries again. He starts driving towards the loop, and after he finishes the first half, if you look carefully, the bike then falls out of the loop. For the rest of the scene, he continues to ride the bike around the second half of the loop anyway. The shot of him riding towards the loop was probably a previous take.