funny how those tiny tiny tiny details are attracting everybody''s attention :-)

pressing ''f'' was simply causing the player''s height to decrease by several units so you end up falling through the terrain without testing for collisions.
But since my collision detection is far from perfect I told also my program to change the player''s height to a bit more than the terrain''s height at that position once the player is falling too deep. (usually that means gravity is pulling you down after collision detection failed, so reseting the player''s Y-Position
could be regarded as an emergency brake)
You end up about where you started falling...

u had 60 frames solid? Damn! what are your system specs
I just have about 40 to 45 on my machine...