An oldie: Halt and Catch Fire: "Halt and Catch Fire, known by its mnemonic HCF, denotes any of several undocumented and semi-mythical computer machine code instructions with destructive side-effects, supposedly included for test purposes on several well-known architectures going as far back as IBM System 360.
The Motorola 6800 microprocessor was the first for which an HCF opcode became widely known. This instruction caused the processor to toggle a subset of the bus lines as rapidly as it could; in some configurations this could actually cause lines to burn up due to bus contention.
See also: killer poke"