Assuming you are in your homedir, if you do e.g. mount /dev/sdxx Doc <TAB> it works, but for umount you need to do umount /ho <TAB> (presuming that was the only thing mounted in /home) as the bash completion for umount only works with absolute paths from root, ~/Doc or Doc won't work.
bash-completion comes from Arch, you should open a bug report at FS. I've removed it completely as it breaks my habits.