Filling a cauldron with anything in creative mode will add an empty bucket to your inventory. However, it only happens when you don't have already an empty bucket, so the bug only occurs once if you fill multiples cauldrons at once without dropping the bucket.

To recreate :

• place 3 cauldrons
• fill the first one with anything, a bucket should pop
• drop it and fill the 2nd cauldron, bucket repop
• fill the 3rd without droping the bucket, nothing special should happen