A bucket of water can be placed on top/beside of a waterloggable block, if said block is already waterlogged.

Replicate:
1) Place down any kind of waterloggable block; For example: any kind of slab (bug cannot be replicated with ladders/sign while standing, works only on sign when crouching)
2) Place water via bucket in the slab: slab will become waterlogged
3) Place water once again in the (now waterlogged) slab: water will be placed on to/beside of the waterlogged slab

Video: https://www.youtube.com/watch?v=VMt4vkNlrtE&feature=youtu.be