E212: Can’t open file for writing

I spend much time in vi/vim. As a good little Sysadmin, I seldom wander around file-systems as root. Of course, I often forget sudo before spending much time editing system files in vi. This leads me to seeing our friendly error message in the post’s title.

In comes this command:
:w !sudo tee %
I’ve probably typed this enough times, that I’ll never forget it. That being said, having it lying around here will ensure that.

Here’s a great explanation of what the heck it’s doing: 
http://stackoverflow.com/questions/2600783/how-does-the-vim-write-with-sudo-trick-work