I've passed a lot of my first 10 professional years fixing bugs for a information security company. One of them was pretty hard to find out because it was inside the boot of a specific IBM machine in a specific version of a HD encryption bootstrap. The sympton was the machine could be turned on normally, but in the second boot we got just the black screen after the BIOS check forever. Oddly enough, in the third boot things got just right, freezed again in fourth boot and so on.

The problem with boot code is that the debug process cannot interfere with the boot memory itself or the bug won't show up. So I got to make one simple change in the assembly bootstrap, assemble the code, boot with a floppy, access a raw disk editor and replace the old bytes for the new test, boot twice and see what happens (mostly some chars printed on screen).

After one week focused the problem decided to show up. It was a micro misplaced bit operation in the blowfish algorithm (our version in assembly) that swap one or two bytes in a specific sector in the disk. This happened just in this machine with this HD and this version of O.S. because in the boot process some code in the O.S. bootstrap made a write operation in the disk just in this "wrong sector". It was not hard to fix this problem, but to find it for the first place, and after this system already running in thousands of different machines for a couple years without a clue.

It was a pretty nice week in the end.