On level 4 once you've got past the razors and the pit of spikes,
 the last block above the spikes will fall down.  Walk back to the
 edge of the pit (so you've got a good run up) and then jump as far
 as you can into the swamp.  Keep your Joystick (or keyboard) in the
 'jump' position while you fly across the swamp to the other side
 without a single scratch.