Are there any hardware-level examples similar to this? I mean some sort of "evil CPU": even if all the software is clean, it will act differently on some input

Are there any hardware-level examples similar to this? I mean some sort of "evil CPU": even if all the software is clean, it will act differently on some inputs (read: some pattern of instructions) to produce output in a way that it's indistinguishable at any user-level "views" without a logic analyzer analyzing all the bits in and out the CPU in physical level as electrical signals which is beyond reach practically, while still performing some evil tasks. A similar idea can be expanded to an "evil RAM" or "evil bus" but you probably get it. I'm pretty sure there are already backdoors already in many CPUs that we aren't aware of, but I was wondering if this particular type of attack has ever been spotted in the wild.