Q: Why Cannot chdir into mountpoint? My os is debian8.
cat /etc/group |grep 'debian8'
root:x:0:debian8
debian8:x:1000:

debian8 is the member of root group.
cat /etc/sudoers
root    ALL=(ALL:ALL) ALL
%sudo   ALL=(ALL:ALL) ALL
debian8  ALL=(ALL:ALL)ALL

debian8 can execute any commands as root.
root@hwy:/home/debian8# find / -name 'vimrc'
/home/debian8/openwrt/feeds/packages/utils/vim/files/vimrc
/root/openwrt/feeds/packages/utils/vim/files/vimrc
/usr/share/vim/vimrc
/etc/vim/bundle/Vundle.vim/test/vimrc
/etc/vim/bundle/vundle/test/vimrc
/etc/vim/vimrc

No Cannot chdir into mountpoint  output.    
root@hwy:/home/debian8# sudo find / -name 'vimrc'
Cannot chdir into mountpoint.
/home/debian8/openwrt/feeds/packages/utils/vim/files/vimrc
/root/openwrt/feeds/packages/utils/vim/files/vimrc
/usr/share/vim/vimrc
/etc/vim/bundle/Vundle.vim/test/vimrc
/etc/vim/bundle/vundle/test/vimrc
/etc/vim/vimrc
Cannot chdir into mountpoint.
root@hwy:/home/debian8# su debian8
debian8@hwy:~$ sudo find / -name 'vimrc'
[sudo] password for debian8:
Cannot chdir into mountpoint.
/home/debian8/openwrt/feeds/packages/utils/vim/files/vimrc
/root/openwrt/feeds/packages/utils/vim/files/vimrc
/usr/share/vim/vimrc
/etc/vim/bundle/Vundle.vim/test/vimrc
/etc/vim/bundle/vundle/test/vimrc
/etc/vim/vimrc
Cannot chdir into mountpoint.

Cannot chdir into mountpoint  output for the last two situation.
Why there  are two lines output  Cannot chdir into mountpoint. ?
@Julie,here is what i get .    
debian8@hwy:~$ cd  /tmp
debian8@hwy:/tmp$ cd

Nothing output.
debian8@hwy:~$ cd /tmp
debian8@hwy:/tmp$ sudo bash -c "cd /"
Cannot chdir into mountpoint.
Cannot chdir into mountpoint.    

Now there  are two lines output  Cannot chdir into mountpoint. 
root@hwy:~# cd /tmp
root@hwy:/tmp# sudo su -;
Signature not found in user keyring
Perhaps try the interactive 'ecryptfs-mount-private'
Signature not found in user keyring
Perhaps try the interactive 'ecryptfs-mount-private'


A: May be you did setup ecryptfs with command ecryptfs-setup-private under root?
In that case the system created /root/Private /root/.Private and /root/.ecryptfs
In that case all you need is to carefully undo it:


*

*In a terminal became root again:


su


*Now, being root type:


ecryptfs-setup-private --undo


*Follow the steps that you get from executing the command below


Please, take care of NOT substitute root by your user name unless you want to delete your user data and be without access to your user forever. 
Your user is NOT root !! You need to do these steps ONLY with root !!

A: grochmal tells the answer.
find / -mount -name vimrc 

It is some security layer. 
