People who exchange telephone numbers with people they hate, never talk to (and never will), or are enemies with. It makes no logical sense yet there's always one enemy-exchanger in a crowd. If you see one, either:
1. Start yelling at them
2. Beat them up
3. Call your nearest mental help facility
4. Run for your life
STAY AWAY FROM THESE PEOPLE FOR THEY ARE MOST LIKELY PSYCHOPATHS.