An invalid 68030 instruction accidentally allowed the Mac Classic II to boot

Article note: This is the niftiest, most deeply technical esoterica I've seen in a while. Apple has an invalid instruction in the ROM of the Classic II (ca.1991), whose undocumented behavior on real hardware is necessary to compute an address during the boot process. Discovered because emulators that implement the documented instruction set crash, either on the invalid instruction or on the next instruction because the side-effect didn't happen.