I had a samba/cifs share mounted, but sent my local computer to standby. After returning from standby, the samba share was listed as still being mounted, but wasn't.
Trying to unmount/remount it, gave me the following error message:
In a German linux forum, it's claimed to be a kernel bug with updating /etc/mtab, and according to another forum entry on whirlpool.net.au, the listing returned by "mount" is simply outputting /etc/mtab.This utility only unmounts cifs filesystems.
[SOLUTION]
So, I've edited /etc/mtab and removed the leftover smb/cifs mount line manually. Now everything was nice and clean again, I did not have to reboot and could mount the share as usual.