Hopper or dropper placed on side of furnace can put multiple (16) empty buckets into furnace, after "smelting" sponge they turn into one bucket with water.
Linked issues
Comments 5

The fix from MC-86007 should be good for this. Just enable back putting multiple empty buckets into furnace by hand, and if there more than one, after drying sponge drop bucket with water, leaving one less in furnace. When there's only one, change it to bucket with water.
Probably fixed/invalid next week with the fix of MC-7222.

Not fixed in 15w44a.
I can still put multiple empty buckets to furnace with hopper/dropper facing it from side/I cant place many empty buckets by hand and they are replaced into one bucket with water after smelting sponge.
Reopen please.
Fixed for future version, see MC-7222.
Confirmed.