A bug in the Pentium-class CPUs by Intel.
When the Pentium receives the instruction F0 0F C7 C8 (hexadecimal), it takes down the system.
I personally don't speak machine code, nor assembly, but from what I can make of it, this instructions generates an error on the CPU that would normally cause a program to crash, to prevent harm. In this case, the program is non-existent, and the CPU will crash itself.
Most operating systems have already "fixed" this bug, by working around it.
Here's a snippet of C-code, to try it out, and to show how easy it is to crash a system, if it is badly designed.
char x [5] = { 0xf0, 0x0f, 0xc7, 0xc8 };
main ()
{
void (*f)() = x;
f();
}