"Thanks,You're Fired!"

A man is leaving on a business trip and stops by his office on the way to the airport. The night watchman stops him and says, "Sir, don't take that flight. I had a dream last night that your plane would crash and everyone would die!" The business man cancels his trip and sure enough, the plane crashes, killing all the passengers. The man gives his watchman a $10,000 reward for saving his life, then fires him. Why?

That's why! 

Source 

This Is Not Allowed On Duty

What exactly happened? 

The man awarded $10000 obviously for saving his life. The duty of night watchman is to keep watch on office at night not to sleep & dream.

That's why he fires the watchman.