Several cannibals were recently hired by a big corporation.  "You are
all part of our team now," said the HR Rep during the welcoming
briefing.  "You get all the usual benefits and you can go to the
cafeteria to eat, but please don't eat any of the other employees." 

The cannibals promised. 

Four weeks later their boss remarked, "You're all working very hard,
and I'm satisfied with you.  However, one of our secretaries has
disappeared.  Do any of you know what has happened to her?" 

The cannibals all shook their heads no. 

After the boss had left, the leader of the cannibals said to the
others, "Which one of you idiots ate the secretary?"  A hand raised
hesitantly, to which the leader of the cannibals continued,  "You
fool! for four weeks we've been eating managers and no one noticed
anything....but nooooooo, you had to go and eat the secretary!"