When you have 2 or more empty buckets, if you try to fill one while your inventory is full, it will drop the bucket with water/lava but also keep the empty bucket, 

for example you have 3 buckets, you fill one with water but your inventory full, you drop 1 bucket of water but you keep having 3 empty buckets

link to showcase: https://1drv.ms/v/s!AkRfDG4LkBYhgS7brac2OiXPBH_T?e=uEFsRJ