The Lie
This may sound ridiculous, but it has happened so many times in practice, that it has earned a place in the Horror Hall of Fame.
A tasker will often give an outright lie as tasking. I once worked five viewers for a full five sessions each, getting nothing but garbage and contradictions, only to find out that the tasker had tasked us to describe facts about a crime which had never occurred. When confronted, the tasker replied with a straight face, "If you were really psychic, you would have known I was lying."