x86 processors beginning with the 80286 will cause a shutdown cycle to occur when a triple fault is encountered.
In modern operating systems, a triple fault is typically caused by a buffer overflow or underflow in a device driver which writes over the interrupt descriptor table (IDT).
In VirtualBox, a triple fault causes a Guru Meditation error to be displayed to the user.
However, the 286 could not revert to the basic 8086-compatible "real mode" without resetting the processor, which can only be done using hardware external to the CPU.
On the IBM AT and compatibles, the documented method of doing this was to use a special function on the Intel 8042 keyboard controller, which would assert the RESET pin of the processor.