Usually appearing in Pits, Milk is an obstacle that kills the player when touched. It first appeared in Magic Key.
Buzz Saws: Edit
Buzz Saws are, like the name suggests, a bladed object that kills the player if they touch it. It was first introduced in the level BuzzSaw.
Bottomless Pits: Edit
Bottomless pits are, like the name suggests, are a hole in the ground that the player can fall into. A few seconds after the player falls down, they die. They first appeared in The Pit.