Biplane Duel 1 & 2

You can destroy the enemy biplane by flying through the edge of the
screen and firing horizontally, then immediately turn around and fly
back through the screen to the other side.  Line up and fly through
your own bullet and the opposition will explode!

This cheat works due to a bug by the programmer.  He detected a kill
by seeing if a bullet from a player had hit any plane, rather than
checking which plane was hit!