This bug appears simply when you push a water source with a piston. It looks like the client think there is water and render it but server part not. You cannot drown in it and it do not flow, and there is many things you can do with it.

• It do not save in the world file and if you reload the chunk, it disappears.
• If you waterlog it and break the block, the water disappears.
• If you climb a water column of bug-affected water sources block in survival, you will take damage.