When you pull an extended slime block with a carpet over it, the carpet doesn't get destroyed, but the carpet item also drops, allowing the duplication of said carpet.
This only works with vertical pistons, horizontal ones don't, but upside-down ones can work if there is a slime block beneath the carpet and the said is linked to a piston
Linked issues
is duplicated by 3
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Comments 13
Fixed in 1.9-pre 1
And it was probably related to: https://bugs.mojang.com/browse/MC-89119
Confirmed.