Revealing mistake: When Indy and Willie jump out of the Obi Wan Nightclub, they fall through several awnings, the idea being that they will break the characters' fall. All well and good. But, if you look very carefully, you can see small taped-up cut marks which the crew made so the stunt men (and women) could effortlessly glide though the awnings.
Revealing mistake: To escape the Obi Wan Nightclub, Indy and Willie jump out the window and fall through several awnings. When they hit the second to last awning, there is a close up on the actors. Just before they hit the awning, there are butt-shaped indents in the fabric from a previous take.
Revealing mistake: When the water floods the mine, some shots of the 3 heroes are an obvious composite of their blue screened image over a set. Problem is, when Indy tells the two to run towards a tunnel on the right with a log blocking the way, Indy and Shorty run through the log as if they were ghosts.
Revealing mistake: In the scene where they are climbing up the broken rope bridge after Indy cuts it, Indy is fighting the bad guy and falls a bit further down the bridge (which is now a ladder). You can see the climbing rope that is attached to Indy's waist (obviously a harness) that stops him from falling all the way. There are several other pieces of rope in the scene because it's a broken rope bridge, but this is quite visible.
Revealing mistake: After Indy escapes from the conveyor belt, he runs on some suspended planks while being shot at. Watch the planks: There's black masking tape where the explosives are (which disappear in later shots). Furthermore, right before he takes out his whip, he grabs a railing where the bullet has just hit, and you can see that the black tape is dangling loose after the spark.