EDIT: This is a duplicate of MC-44352, please ignore.

Place a slime block on the ground, then place another block three blocks above it (so that there are two air blocks between the placed blocks).

Now jump onto the slime block. Note that the screen will now jitter, as if the player was moving up and down very fast. This jittering does not stop until the player exits the slime block. Pressing F3 indicates that the y-value is not changing, but the "g" value (bottom line of the F3 screen, indicates whether the player is touching the ground) is changing.