Linux

Here's some of the more unusual things I've found that screwed me up...

initrd doesn't have drivers for root partition
copy of fstab in initrd doesn't reflect system I'm booting to (this can also cause wrong drivers)
grub.conf doesn't indicate correct /root partition (especially easy to miss when identified by UUID)
grub-install hasn't been run since drives have changed, so it's looking in the wrong place for initrd or kernel.