Package: os-prober Version: 1.44ubuntu1
If the Windows partition contains a /boot/grub/core.img folder (this occurs when beginners install GRUB by mistake in the Windows partition), os-prober won't detect Windows any more.
I believe this is due to the fact that there are 2 folders with similar names ("/Boot" and "/boot"), so os-prober just sees the /boot folder instead of the /Boot or /BOOT folder coming with Windows)
I am using Ubuntu 11.04 32bits, but this bug has been also confirmed with previous os-prober versions.