If you place a stack of items/blocks on the precise edge of a hoppper with a full inventory and then take out one stack/item of the hopper inventory, it takes the amount of items on top of the hopper and places it in hopper inventory but only decreases the items on top with one.
(behaves a bit differently with single items/not full stacks)
When repeating the process with 63 items on top it creates a stack of 63 items inside and takes away one from top, then sucks up one more to fill up the stack inside thereby decreasing with 2.
If you then repeat the process once again it takes away 3 items on top while creating 64 new, then 4... 6... 7... and so on.
Works with any item/block
Duplicate of MC-5998.