Breaking and replacing a floating block extremely quickly, will place the new block floating in the position of the old one, even though there is no surface to place the block on.

HOW TO REPRODUCE:

Step 1: Place a block and make sure it is floating

Step 2: Press both left and right click at the same time.

Step 3: You can see the particles of the old block breaking with the new one being place directly after. Looks like particles coming out of the block and nothing more.

Sorry there is no picture. It's pretty hard to capture an instantaneous action.