Some memory-bound instructions (namely, those
useful for implementing atomic counters) on x86 are 
allowed to take the LOCK prefix. In binary, this
is the "0xf0" byte that precedes the instruction
opcode.

An explanation of legitimate use:
http://www.codemaestro.com/reviews/8
(not definitive but the shortest I could find).

The LOCK prefix for MOV between registers is illegal
and causes the Undefined Opcode trap (#UD). 
Bryan Cantrill explains metnions it here:
http://markmail.org/message/duppnekq6xf3h26e

However, the illegality of LOCK for a certain 
atomic register exchange instruction was not caught
and caused the Pentium processors to enter an 
inconsistent state and freeze. This became
known and the "f0 0f" bug:
http://www.x86.org/errata/dec97/f00fbug.htm