I recall trying to debug a crash in QNX during the mid-90's. I was impressed by the svelte OS that could load from a 3.5" floppy. The failure scenario was coincident with one of my first tasks as a BIOS engineer and it entailed adding some custom error logging in System Management Mode (SMM). Luckily for me it turned out that I had forgotten to save/restore certain general purpose registers around my SMM logic. Fun times. SMM is pretty good at 'breaking' operating systems :)