Yes, we're here again. We have two bloopers from the Indiana Jones franchise, and they're both from the same film! One mistake is careless, two makes you wonder if anybody was really checking. We feel for them with this one. They really were trying hard to ramp up the danger. It would be easy to use a fake cobra for the scene where Indy comes face to face with his worst fear, but the producers used a real one. So now they had to deal with the question of how to keep the actor safe.
They decided to use polished glass to achieve the effect, with the idea being that its see through so if shot from the right angle it would look like Indy really was in mortal danger. Unfortunately, glass reflects light. For example, the light behind the snake that illuminates the scene, meaning we can see the reflection of the snake in the glass. Effect ruined!