Revealing mistake: During the campus chase scene, Mutt skids his bike and turns left to drive under a stone arch. Multiple skid marks are seen on the pavement from previous shots.
Continuity mistake: The gold mask on Orellana's face changes angle between shots.
Continuity mistake: After the monkey chase scene, after Spalko's jeep lands on top of Indy's, Indy, Mutt, Mac and Oxley start to raise, but when the angle changes they're crouched again, plus in a completely different position.
Continuity mistake: Spalko steps on a tree to free herself from the ants, 2 meters away from where a soldier is starting to be eaten. A frame later the soldier is gone, there's not even a small mound where his corpse could be.
Continuity mistake: Oxley takes the skull out of the bag and places it behind his hand to scare the ants away. When the angle changes there's a brief glance of his right hand and the bag is gone.
Continuity mistake: When the aliens kill the Russians, the group is launched out of the temple thanks to a jet of water. Seconds later they're all completely dry.
Continuity mistake: When the group enters the final temple the skull falls into the water. Mutt picks it up, and there's a shot of Mac grabbing some gold. Then back to a wide angle and Mac is repeating the previous movement with a totally different hairstyle.
Revealing mistake: At the end Indy picks up a heavy slab and uses it to knock the carved faces off the wall. He hands it to his friend Mac, then takes it back off him - at this point it's clearly a very light prop.
Continuity mistake: In the wedding scene the bride has open toed shoes when she kisses Indy and begins to exit the pulpit area, but different closed toe shoes as she and Indy depart the church.
Continuity mistake: By the waterfall Indy says "Nobody else can" and then Mutt's wet hair swaps from brushed backwards to messy.
Continuity mistake: After Mutt points at the waterfall, Indy grabs the skull and Mutt leans on him to raise up, yet a frame later from a different angle his hands are away from Indy.
Continuity mistake: When the raft is about to fall through the second waterfall, there's a huge rock to the left which disappears in the wide angle.
Continuity mistake: When Indy removes Orellana's golden mask, the light changes abruptly from very shiny to obscure and mysterious.
Continuity mistake: During the quicksand scene, when Mutt is throwing a snake at Indy, Indy's left (our right) sleeve swaps from covered in sand to spotless between shots.
Continuity mistake: When Mutt and Indy enter the library, a chubby man in a suit holding a brown briefcase runs away, yet appears calmly walking half a second later from a different angle.
Continuity mistake: During the ant scene, the branch that Indy uses to knock out the Russian wasn't there in the first place, it magically appears between shots.
Continuity mistake: After Mutt's bike knocks down the "Better red than dead" banner it falls sideways; yet when the chasing car arrives, the banner is magically up again, ready to fall horizontally on the car's windshield.
Continuity mistake: Mutt's bike passes by a group of strikers in the campus, and heads for the red banner. When the angle changes, a second group of strikers has suddenly appeared out of nowhere next to the banner.
Revealing mistake: When Indy and Mutt ride on the bike in the college gardens, there's a wide aerial angle that reveals that the distance to the red banner is very short, yet in every following angle they're repeating the same route over and over again (3 times at least) to make it look way longer and increase the suspense.
Audio problem: During the chase inside the campus, a car destroys some stone vases and a blonde woman cries in terror while running away, yet her mouth is not moving.
Chosen answer: Because the film had a global release date, and premiered the same day in every country in the world. Obviously, the studio felt this was the best day to do it.
Twotall