Collecting any type of fish (tested with tropical fish, cod, tadpoles, and an axolotl by accident) with an empty bucket causes them to disappear instead of nothing happening. It also causes a bucket to be removed from the stack (if the bucket is stacked) You can however collect it with a water bucket, as shown in the video clip below.

Steps to reproduce:
-Get an empty bucket (NOT a water bucket)
-Attempt to collect a fish using the empty bucket

Expected Result:
Nothing happens (you cant collect fish with an empty bucket)

Observed result:
Fish disappears with sound of water being picked up and a bucket will be removed from the stack (unless there is only one bucket, in which case it wont disappear)

(sorry about the watermark)