Couldn't think of a decent example off the top of my head.
Also, proper term is 'undocumented opcodes' for further searching.
Try
"AXS * (SAX)
AXS ANDs the contents of the A and X registers (without changing the
contents of either register) and stores the result in memory.
AXS does not affect any flags in the processor status register."
Also, proper term is 'undocumented opcodes' for further searching.
Try
"AXS * (SAX) AXS ANDs the contents of the A and X registers (without changing the contents of either register) and stores the result in memory. AXS does not affect any flags in the processor status register."
http://www.ffd2.com/fridge/docs/6502-NMOS.extra.opcodes