When upgrading from 6.0 to 6.1 in a dual-boot system lilo was reconfigured with boot=/dev/hda, while originally was boot=/dev/hda5.
In addition, upgrade does not preserve special configurations in lilo.conf, such as mine 'disk=/dev/sda bios=0x81, disk=/dev/sdb bios=0x80' [yes, it's stupid not to have the lowest SCSI-ID as the boot-disk, but this one is like it is for historic reasons]. This rendered the system unbootable after upgrade. Fortunately I was able to fix it using just the bootable RH61 CD, but the fact that the original lilo.conf wasn't even preserved as lilo.conf.rpmsave made the fix a bit harder, since I didn't remember the correct syntax for those configuration lines...
*** This bug has been marked as a duplicate of 5578 ***