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
