In computer jargon, a killer poke is a method of inducing hardware damage (i.e., actual physical, irreversible damage) on a machine and/or its peripherals by the insertion of invalid values, via e.g. BASIC's POKE command, into a memory-mapped control register. The term is typically used to describe a family of fairly well-known tricks that can overload the analog electronics in the CRT monitors of computers lacking hardware sanity checking (notable examples being the IBM PC and Commodore PET; a similar trick is reported having been done to Atari ST displays).
The PET-specific killer poke is connected to the architecture of that machine's video rasterizer circuits. In early model PETs, writing a certain value to the memory address of a certain I/O register made the machine able to display text on the screen much faster. When the PET range was revamped with updated hardware, it was quickly discovered that performing the old trick on the new hardware led to disastrous behavior by the new video chip, causing it to destroy the PET's integrated CRT monitor.
The TRS-80 Model III had the ability to switch between a 40-character-wide display and an 80-character display. Doing so actuated a relay in the video hardware, and was accomplished by writing to a specific memory-mapped control register. Programs that repeatedly switched between 40 and 80 character modes at high speed (either on purpose or accidentally) could permanently damage the video hardware. While this is not a single "killer poke", it demonstrates a software failure mode that could permanently damage the hardware.
The BBC Micro from Acorn Computers, popular in British schools, had a built in relay for controlling an external tape recorder. Many 'computer studies' classes in the late 1980's resounded to the buzz of smart-alec schoolkids toggling the motor control relay in a tight loop, which reduced the relay's longevity. (This example is not unique to the BBC Micro; other computers of the cassette-recorder-storage era, such as the TRS-80 Model 100, could have the tape relay similarly controlled.)
Any system that meets Popek and Goldberg virtualization requirements can be made immune to any killer poke entirely by software means. The reason is that the VMM is required to intercept all the privileged instructions, which include sensitive and dangerous ones such as POKE, making it possible to then filter dangerous instances of that instruction.