Liquids can be placed on the back of rapidly extending pistons while you are in a boat..
How to reproduce:
1) Place down a piston facing you.
2) Connect it to a fast redstone clock (but make sure there is air directly behind the piston).
3) Place a boat down in front of the piston and enter.
4) Aim at the middle of the piston, hold a water.lava bucket in your hand and hold right click (or whatever key you use to place blocks).
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I've tested this and you don't need to place liquids, blocks work too. Also you don't need a boat. I believe it's because of the piston disappearing every time it extends and shrinks back.