Ceiling Hover

In 1.8, this is the only setup that can be used to perform a Ceiling Hover (1.8125bc with slime underneath)

Ceiling Hover is a glitch that makes the player "hover" between a ceiling and a bouncy block (slime, or beds since 1.12). It can be performed by jumping under a ceiling such that the block 2.001b below the ceiling has bouncing properties.

Explanation

Whenever the Player collides vertically with a block (floor or ceiling), the game will apply collision physics.

For almost every block, that just means setting the Player's vertical speed to 0.

/* in class Block */
public void onVerticalCollision(Entity entityIn)
{
    entityIn.motionY = 0.0D;
}

There is one exception: Slime Blocks (and Beds in 1.12+).

/* in class BlockSlime */
public void onVerticalCollision(Entity entityIn)
{
    if (entityIn.isSneaking())
        super.onVerticalCollision(entityIn);
    else if (entityIn.motionY < 0.0D)
        entityIn.motionY = -entityIn.motionY;
}

• If the Player is sneaking, it's treated as a regular collision (Vertical motion is set to 0)
• Otherwise, it checks if the Player's speed is negative, then inverts it.

Note that nothing would happen if the Player is neither sneaking or moving at negative speed.

In fact, that's exactly what happens with this glitch.

When the game detects a vertical collision, it will consider the block 0.2m under the Player's position to apply collision physics (yes, even for ceiling collisions...)

Let's review the steps for the 1.8125bc Ceiling Hover setup:

1. Jumping under 1.8125bc applies vertical collision with the block 0.2m below the Player (at ground level).
2. If that block is a slime block, then vertical speed won't be set to 0, and the Player remains suspended under the ceiling.
3. Repeat step (2) until the Player's vertical speed becomes negative due to gravity.

Note: This glitch happens for a few ticks, but with Jump Boost it would take longer. Ceiling Hover can be interrupted at any time by sneaking.

Consequences

This glitch is pretty insignificant: it isn't game-breaking in any way, and requires a very specific setup to perform.