Comment by atakan_gurkan

This reminds me of the "illegal opcodes" in Commodore 64. I believe they were even present in their entirety in Commodore 128's (which uses 8502, not 6502 as its main CPU) C64 emulation mode. If someone knows how or why they remained part of 6502's instruction set over C64's lifetime, I really would like to know. I suspect, it was deliberate since "customers relied on them".