Continuity mistake: Indy's raft lands on the water, and he is ducking but rises up and rows with his hands for a couple of seconds. Then a frame later he is still ducking and rises up again.
Sacha
Continuity mistake: When Mola Ram is climbing the bridge, Willie and Short stand together; half a second later Mola Ram falls and Short has managed to climb up and move apart to let him pass by. Then a nano-second later, Mola Ram grabs onto a piece a wood and Short and Willie manage to stand together again. (01:41:15)
Sacha Continuity mistake: On the bridge, an arrow suddenly appears to the right of Indy's foot seconds before any arrows are shot at him. (01:42:55)
Sacha 
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