A K fail is when you play guildwars with someone, (who has been playing since beta) and he has no clue as to what he is doing. He doesn't know how to do the mission or bonus. He constantly runs to the front to commit suicide, double aggro's and uses the worst rez in the game. (rebirth) He also has fail heroes with fail builds.
K Fail is the new "noob".
Friend: Hey want to come do a mission with me?
You: As long as you don't pull a K Fail.