Riddles, brain puzzles and mathematical problems

[BAndrew]

A wealthy art collector had planned an air trip abroad for a major auction. The morning of the same day but a few hours before departure, the night watchman calls him and tells him that last night he had seen a very bad dream: The flight will fall and all the passengers would die. The collector tried to calm him down, but he insisted that the dream seemed very real, and finally persuaded him to cancel his flight. And indeed a few hours later that flight crashed with tragic consequences for all occupants. The collector immediately calls the watchman and thanks him for saving his life, but announces that unfortunately he has to fire him! Why?