OK, this is bizarre. I placed a slab, put water in it, and in some( or most) angles, you can right-click the slab with the bucket and congrats, you made your self an infinite water source, from ONE water source.
MC-128721
Infinite water using 1 water bucket and 1 slab
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Just saying, I first thought this might be a new feature, since I saw the water flowing through the slab, but when I tested it with fences... well, it doesn't work, obviously. So this is CLEARLY a bug, NOT a feature.