I'm using PCManFM 1.1.2 and I wanted to make some tests with ntfs-3g and mounted a ntfs partition and then killed the mount.ntfs process. This caused some weird behavior on the /mnt directory. For example ls does still show the directory but it noticed that something is wrong:
root@ubuntu:/# ls -a
ls: cannot access mnt: Transport endpoint is not connected
. .. bin boot dev etc home initrd.img lib lib32 lib64 libx32 lost+found media mnt opt proc root run sbin srv sys tmp usr var virtualbox vmlinuz wine
But PCManFM isn't showing the directory anymore.