Cobb is considered the greatest actor on Earth, but wears a paper bag over his head, disguising his face in all his roles and public appearances.
Bender manages to sneak past Cobb's fungus-based guard dog and takes a picture of him without his paper bag.
Bender develops the photo of Cobb and shows it to Fry, Amy, and Hermes, and the sight of it causes their lifeforce to violently escape their bodies, leaving them as nothing but empty, deflated husks.
They suggest he perform the death scene in Romeo and Juliet, and Calculon recognizes that the only way to ensure his full believability is to take food coloring, which is fatal to robots, and die.
To stop him, Bender attempts to show him the picture of his own face, reasoning that as a biological creature, Cobb should likewise be destroyed by his own image.