Continuity mistake: When the pilots jump off the plane the flowers on Willie's hair swap positions for a brief moment.
Continuity mistake: While hanging off the bridge, Indy's shirt keeps changing from light to dark, and from very dirty to slightly dirty all the time.
Continuity mistake: The raft arrives in a big place with no snow, yet from the opposite front angle there's snow all around.
Continuity mistake: When Willie falls from the elephant into the puddle of water, her hair varies from very wet at the bottom to mostly dry from each different camera angle.
Continuity mistake: In the mine cart, when Indy hits a tank with a shovel, the Thuggees behind raise their arms, yet a frame later, from a wider angle the arms are lowered.
Continuity mistake: When Willie and Short get to the bridge there's a big tree on the left. In subsequent shots it's gone.
Continuity mistake: At the start of the mine car chase look at the length of the braking poles on the side of the car. When they leave the main cave the roof lowers and the braking poles are slightly higher than the side of the car but were a lot longer to start with.
Continuity mistake: When that guy is lowered into the lava pit, the cage is level horizontally. When the empty cage is raised again, it's slightly at an angle.
Continuity mistake: During the mine cart chase, Indy hits one of the switches, putting it from the forward to the back position. When the guards in the mine cart get close, they shoot it, putting it from the front position to the back position again.
Continuity mistake: As the Thugee ceremony comes to an end, you see the stage hands exit and the spectators get up and start to leave. The next scene shows Short round looking over the edge and the stage hands are still on the stage, just starting to leave and some of the spectators are still in the process of standing up to leave.
Continuity mistake: When Willie walks out of her bedroom while Indy is fighting the assassin, the left sleeve on her pyjamas changes from rolled down to rolled up between shots.
Continuity mistake: In the mine cart, after Willie cries "Hurry" there's an aerial shot of the cart and Short Round has vanished.
Continuity mistake: When Indy is stopping the mine cart with his foot Willie's hand is on top of Indy's or not depending on the shot. This swaps back and forth.
Continuity mistake: When Indy falls inside the mine cart, the protruding log disappears. A shot later it reappears, but when Indy hits the railroad switch with the shovel it disappears again.
Continuity mistake: Before the first sacrifice victim is being lowered into the lava pit, Mola Ram is standing on the edge of the pit, but in a few shots later he moves some feet away.
Continuity mistake: When Indy, Willie and the kid are leaving the village riding the elephants the sun changes between shots.
Continuity mistake: On the ladder, when Mola Ram tries to grab Indy's heart, Mola's hands and Indy's chest are very sweaty and dirty in the close-ups, but dry and quite clean in the wide angles.
Continuity mistake: On the ladder, when Willie and Shorty kick Mola Ram, Willie is positioned higher than Shorty, but when the angle changes they're both parallel.
Continuity mistake: When the thug jumps on Indy's mine cart, Indy's hat gets loose, but in the following shot it's perfectly tucked in.
Continuity mistake: After Shorty is saved from the thugs' mine cart, a baddie jumps from an elevated cart situated on the right side, but when the angle changes he is coming from the left side. Also, Indy is looking sideways instead of upwards, as if the bad guy was parallel to him.
Chosen answer: It was actually George Lucas who wrote the story, made it a prequel, and has stated it was because he didn't want the Nazis to be the villains again. The idea most likely seems if it wasn't a prequel, the Nazis could still be after Jones, even in China or India. But alas, there is really no other insight as to Lucas' prequel decision.
Bishop73