Placing water source inside a trapdoor (opened) while it has a ladder underneath it. You cannot pick the water source back up.

When you create a stair, place a trapdoor above it which is closed. Place a water source block from a bucket of water inside the trapdoor. Now you cannot pick the water source back up.

You can only pick it up when the ladder beneath it is removed and replaced with a solid block. 

 

Very annoying issue when you just want to open a trap door and you place the water source from a bucket on accident.