Yesterday I performed an automatic security update suggested by the update manager on my virtualized (with VirtualBox on a Windows 7 host) Ubuntu 10.10 installation.
The update somehow failed and left me with an unbootable system. When I try to boot, I am told that various folders, files, and what not are missing. Then the system drops into a busybox and leaves me with an (initramfs) prompt.
This happens with all kernels I get offered by GRUB, although the error messages are quite different from kernel to kernel.