While standing on a slime block, without bouncing or moving with a piston, the screen vibrates, as if the slime block is trying to launch the player at 0 m/s.

Possible fix: Add a condition to slime block making a player jump, that if the player's vertical velocity != 0, jump will happen. Example:

if ( player_velocity_y != 0 )

Video proof: http://www.youtube.com/watch?v=vXAlZ-o039Y (Watch at high resolution)
While Mumbo is standing on a slime block, his screen vibrates slightly, but while he's standing on any other block vibrate stops.