12. The Sheet Of Glass
One of the defining character traits of Indiana Jones as a character is that he hates snakes. We had to wait several films to learn why this is the case, but happily neither Harrison Ford nor Steven Spielberg share Indy's ophidiophobia.
Even so, for the sequence in which Indy is awkwardly dropped into the Well of the Souls and finds himself face to face with a cobra, which is poised and ready to strike, you can faintly see the reflection of a sheet of glass between Harrison Ford and the venomous reptile. Apparently, the cobra even sprayed its venom onto the glass when it saw Ford.
The reflection was removed in the 2003 digitally enhanced re-release of the film, although it would be interesting to see it in the film as a testament to the use of practical effects in the making of this classic.