There are a few non-mandatory enemies[NME] (meaning that you don't need to kill them to clear the level) in the game that don't have hitboxes. This means that if desired, you can go straight through them to get kills, which is sometimes faster.

List of known hitbox-less NME: Honeycombs ( World 5 ) Ice Cubes ( World 4 ) "Tech honeycombs" ( in level 7-6 ) Bones ( World 8 ) Bubbles ( World 2 )