This will be a collection of scenes that are in my own opinion the most unbelievable from each movie. I am not counting KotCS because I have only got to see it once.
I also am not going to count the supernatural scenes, as we all know that that would win everytime.
Raiders of the Lost Ark: The snakes in The Well of the Souls. How did all of those snakes survive after countless eons down in that dark pit? There was no sunlight to warm them on cool nights, there was no way for them to feed, and where were they to lay thier eggs/birth their young?
Indiana spies them slipping in from a hole in the wall. Where were they coming from?
Temple of Doom: Two things stick out in my mind.
One is the mining cart jump. It would be like a one in one-trillion chance that that cart would land so perfect onto the rails.
The cart was even going uphill at the time of the jump, so it should have over-jumped if anything.
Two, the water.
The tank that Mola Ram had tipped over was big, but it was not so big that the water from it would cascade down hundreds of branching paths, go through molten lava, and then spew with such force out of the portal at the cliffside.
Last Crusade: I am not going to get into the petrol in the water issue. But, I will say that besides that scene the flying scene is the most unrealistic.
We have Indiana pioleting a plane against not one, but TWO German barons, and we are expected to believe that Indiana Jones zip and fly and dodge those vetern's attacks?
Rubbish.