The PIC18 instruction set has this property. Most instructions take up a single 16-bit instruction word, but the few ones that take up 2 words all have the 0b11

The PIC18 instruction set has this property. Most instructions take up a single 16-bit instruction word, but the few ones that take up 2 words all have the 0b1111 in the high bits of the second word, which would execute as a NOP if branched into. This encoding is presumably a consequence of the instruction set containing conditional skip instructions. That is, the skip instructions don't have to parse the instruction stream for 2-word instructions but can always just skip a single instruction word and let the second word execute as a NOP.