It is very annoying to see message "E212: Can't open file for writing" when trying to save file.
Hopefully there is a simple solution (a bit longer but without "W12 Warning"):
:execute ':w !sudo tee % > /dev/null' | :edit!
Look at commandlinefu.com for more solutions like :W command below:
:command W :execute ':silent w !sudo tee % > /dev/null' | :edit!