One cool thing about this is that there are places where old compilers feared to tread, on the 6502 for example any opcode (apart from $A2, LDX #imm) ending in

One cool thing about this is that there are places where old compilers feared to tread, on the 6502 for example any opcode (apart from $A2, LDX #imm) ending in 2 would be a JAM instruction - CPU just stops. The later CMOS versions still did that for most of the $x2 instructions but some were converted to NOP. So if you're implementing a 6502 in an FPGA, and you want to add (say) stack-relative addressing, there's a nice set of opcodes that you know won't ever be in conflict with the usual ones, because any 6502 ever issuing it would immediately halt (but not catch fire).