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.

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