I remember the knowledge/idea of illegal opcodes for the C64 slowly making their way through the hacker/cracker community. Like secret knowledge being distilled back then via BBS
For example Store A/X/Y to any unused/scratch Zero Page location.
Or condition branch to the next instruction using a condition known to be true .. BCC/BCS is often the most convenient. Or just JMP to the next instruction, though it's 3 bytes of code instead of 2.
INC/DEC of a scratch Zero Page location is useful for a 5 cycle delay in 2 bytes of code if you don't mind changing NZ flags, but the store/branch/jump alternatives don't even do that.