There are some bugs with "enable sleeping" turned on. Occasionally an object that is sleeping seems to fail to have external forces act upon it. For example, you can end up with a sleeping object sitting on top of another object, and when the object falls out from under it, the sleeping object stays stuck in midair. I can reliably make this happen by toggling "enable sleeping" on and off (example: load up "wrecking ball", stop the wrecking ball before it hits the pile, turn on "enable sleeping", wait for all the blocks in the pile to go to sleep, then turn "enable sleeping" back off and you now have a stack of immovable blocks which the wrecking ball bounces off of, and which will hover if you remove the bricks below them), but I saw the bug occur even when I wasn't flipping this setting back and forth.
Enabling sleeping is a difficult thing to actually do, especially in a stack of bodies. You have to enforce a minimum movement threshold over a series of frames usually, and group bodies together to go to sleep, and also wake the same group up when there's motion. It's not always easy to do
