If you try to empty a petal apothecary while having a stack of 2+ empty buckets, you get and extra filled bucket back while keeping your stack of empty buckets, if you only have one bucket it works properly though.

@https://github.com/Vazkii/Botania/blob/master/src/main/java/vazkii/botania/common/block/BlockAltar.java on line 115

If you look at that line it simply adds the bucket back without removing one from the player's stack