It! The Terror from Beyond Space

The mask of the monster suit was altered considerably. When Ray Corrigan was fitted for the suit, the mask was initially too tight. Paul Blaisdell, who made the suit, had to remove and rebuild the monster's lower jaw so the mask would fit better. Unfortunately, Corrigan's chin stuck out through the opening made in the mask. Blaisdell made up his chin to look like the monster's tongue. The mask's original eyes (large and catlike, a Blaisdell trademark) were also removed; the eyes you see behind the mask are actually Corrigan's.

Van Heusen opens the reactor shield to try to kill the monster, but it smashes through the door to escape. He doesn't reseal the shield, though. The adjoining space, the one with the man with the broken leg, should have been flooded with deadly radiation ("enough to kill a hundred men" as Van Heusen notes).

Edited into Invisible Invaders (1959)