Basilisk (thing)

Also a name David Langford gave to images whose mere seeing might cause damage in human brain. Any basilisks are not known to exist, but in theory they could be possible. From comp.basilisk FAQ:

He imagined the human mind as a formal, deterministic computational system - a system that, as predicted by a variant of Gödel's Theorem in mathematics, can be crashed by thoughts that the mind is physically or logically incapable of thinking. The Logical Imaging Technique presents such a thought in purely visual form as a basilisk image which our optic nerves can't help but accept. The result is disastrous, like a software stealth-virus smuggled into the brain.

Some basilisk-related appearances in fiction:

• Fred Hoyle's Black Cloud: an extraterrestial lifeform tried to upload galactic encyclopedia into human brain at very high bandwidth using suitable images to hack human brain, but the subjects died to internal inconsistencies in knowledge database
• J. B. Priestley's The Shapes of Sleep: shapes to evoke certain emotions in advertising
• Piers Anthony's Macroscope: as ultimate security measure in communications, unauthorized viewers would see a sequence of images that erased their minds
• Monty Python's Flying Circus: a joke that caused everyone hearing it to laugh to death.
• Babylon 5: PSI corps controlled Michael Garibaldi's mind with suitable images sent over babcom.
• Neal Stephenson's Snow Crash also brushes the subject, according to muted.

source: comp.basilisk FAQ, www.nature.com/nature/journal/v402/n6761/full/402465a0_r.html, except for the babylon 5 part =)