If I put myself in the shoes of someone truly malicious, I would request a backdoor in the chip implementation itself. Hardware is hard. Once you put it in the customer's hands they can't change much. Hidden opcodes or combinations/uses of legitimate opcodes that trigger undefined behaviors on the platform as a whole are the way to go. Firmware on the other hand can be patched or replaced.
It's probably an Easter egg or an undocumented workaround for something.
It's probably an Easter egg or an undocumented workaround for something.