The term 'zwtf' is used when an inflationary use of the term wtf characterizes one's language use in chatrooms and/or emails resulting in a devaluation of the original wtf.
When it then comes to situations which seem really wtf you need to find a term stating the actual urgency and significance of the 'wtf-situation'.
For such cases the letter 'Z' puts much more emphasis on your exclamation of wtf.
Dude1: Hey there is another 'kill the kitten game' on newgrounds!
Dude1: And I learned in school that Hitler was a bad guy.
Dude1: Your mom just killed Chuck Norris and Mr. T simultaneously.
wtf with an extra letter made in an accident to push the shift key, kind of like the word zOMG but in this case, it's zWTF
ZWTF THAT'S NOT A FISH!