If you are standing in a cauldron with a trapdoor on top of it and you close the trapdoor then your character will perform the swimming animation, even if it is empty.
View the attachment I included below to see how to reproduce.
MC-160809
Players are able to swim in cauldrons using trapdoor
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Is this really a bug? trapdoors cause the player to enter crawl/crouch stance. so considering cauldrons may be filled with water, the swim animation makes sense when filled. it would only be a bug if the animation was swim when empty.