Bucket Duplication by clicking on Fish with a stack of buckets

Essentially hold a stack of empty buckets. I was able to get it to happen with any number of stacked buckets, but I first found it with the max stack of 16.

Hold the stack of buckets, go up to a fish in water and try to catch it with your empty buckets. Your stack will be duplicated and you will then have two stacks of 15 and 14 empty buckets.