What is the most irritating bug in your OS?

[Bender]

Just asking,
For me :
When I try to exit the CLI the OS crashes

lol...........

[Kazinsal]

Presently, certain versions of GRUB2 seem to be either passing a mangled multiboot struct to my kernel or failing to load modules properly, so my initrd doesn't work on various GRUB2 installations, while the next one I try might work just fine.

[qw]

Lately I was rewriting my boot loader. I wanted it to walk the FAT chain so it loads the kernel even if it is fragmented (like any decent FAT driver does). It didn't work and I just couldn't find why. Assuming I made a mistake in the loaders logic, I rewrote it two times. In the end it turned out that the cause was something very different: I had loaded BP, which is used to access variables, with the wrong base address. About six hours spent on such a silly mistake.