There are undocumented opcodes that still perform useful operations on the NMOS 6502, for example loading the A and X registers simultaneously, decrementing and comparing memory, along with a variety of more exotic instructions that can replace quite long sequences of equivalent "valid" opcodes. These are NOPs on the 65c02.

I know little about the Apple 2 series, but in quite some C64 software, these operations are used wherever they can save a few cycles. Mostly demos.