HCF, noun: abbreviation of Halt and Catch Fire, a playful and semi (but not wholly) mythical machine code instruction. The instruction started life in the 1960s as a fictitious undocumented feature, but has since been used as a general way to refer to undocumented machine instructions or instructions that have catastrophic results.
Motorola's MC6800 was the first microprocessor to have a widely known HCF instruction (0xDD). It was intended as a self-test feature and caused several bus lines to toggle rapidly and, barring a reset, without interruption. This could allegedly cause lines to burn, although no incidents of fire were ever reported.