The NSA Instruction (2019)
Back in the mid-2000s I worked at a company that made their own (MIPS-based) chips. NSA was one of our customers - supposedly the "defense" who could be considered the good side of NSA compared to the 10x larger "offense" but still. As we were planning for our second generation, they offered quite a bit of money if we'd implement a "sheep and goats" instruction. It would take two operands: an input and a mask. The masked-in bits of the input (the "sheep") would be packed toward the MSB of the output, while the masked-out bits (the "goats") would be packed toward the LSB. We had a lot of people on staff with serious chops in all sorts of math including cryptography, but none of them could identify an algorithm that would benefit from having such an instruction (as distinct from more conventional range-based bitfield instructions). Since the company went under shortly afterward, it remained a mystery. I still wonder about it.