Halt and Catch Fire. This somewhat fictional operation would HLT the CPU and then toggle the bus line voltages as fast and quickly as possible. If you had any dust along these lines, it would sometimes ignite due to the heat that said toggling would create. Apparently HCF was seen on some early Motorola 68k CPUs.

Also used to describe anything halting and catching fire. For example, you could tell some early Commodore disk drive heads to seek past their physical limitation, thus causing it to crash against the side and die a horrible death involving smoke and melting plastic. This was used as a form of attack on some very early Commodore-based BBSs.

Used more as a verb nowadays; "Oh my god, that truck just HCF'd!"