If a piston is extended horizontally with redstone block behind it, and one on top of that redstone block, if you break the bottom then the top, piston will stay extended without anything powering it. (See pictures)
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Because pistons can be powered diagonally, the power source can be removed without updating the piston and causing it to retract. (See MC-108)
It seems strange that they are intended to work counter-intuitively, but okay I guess. BTW, it only does this with Redstone block, and not redstone torch, but that's probably known as well.