INTEL 8080 Chip

"M.Shima [21] was the 8080 project manager under Faggin. My specification used all 256 operation codes, and 12th from the bottom of my list was an obscure instruction (XTHL) for exchanging the top of stack with the HL register pair. This instruction required five memory cycles to execute, and would be used to pass arguments to subroutines. I carefully explained each instruction to Shima, whose patience was tested as I detailed the XTHL operation. He drew a line under this instruction, and declared:

"No more."

This is why the last 12 instructions were never implemented and why there was room in the instructions set for the 8085 microprocessor's added instructions [23]."

Stanley Mazor "The History of the Microcomputer - Invention  and Evolution"  

Intel 8080 Die - Courtesy of Stanley Mazor 

The X-Number World