The invalid 68030 instruction that accidentally allowed the Mac Classic II to successfully boot up

A bug in the ROM for the Macintosh II was recently discovered that causes a crash when booting in 32-bit mode. Doug Brown discovered and documented the bug while playing with the MAME debugger. Why did it never show up before? It seems a quirk in Motorola’s 68030 CPU inadvertently fixes it when executing an illegal instruction that shouldn’t have been executed in the first place.

I was starting to believe something that sounded almost too crazy to be true: Apple had an out-of-bounds jump bug in the Classic II’s ROM that should have caused a Sad Mac during boot, but they had no idea the bug was there because the 68030 was accidentally fixing the value of A1 by executing an undocumented instruction. How could I prove that my theory was correct?

By buying a Classic II and hacking the ROM in order to see exactly what is happening on hardware, of course!
↫ Doug Brown

What follows is his process for investigating the room on emulated hardware, and then testing it on actual hardware.