Placing a water bucket directly onto a facedown trapdoor while crouched and standing on top of the trapdoor will result in the following:

-creation of a "ghost" water block

-water in water bucket is not spent

-opening of the trapdoor

The "ghost" water block will not flow, is only visible to the player who placed the "ghost" water block, will not prevent fall damage, but will allow for use of the riptide trident enchantment action. 

To remove the "ghost" water block, break the trapdoor.

To prevent this, first open the trapdoor, then place the water bucket onto the block below the trapdoor.

This bug has existed for several updates.