Standing directly underneath any kind of trapdoor and closing it will force the player into a swimming animation, even if there is no water near them. "Swimming" out from below the trapdoor will return the player to a normal state.

Steps to reproduce:

• Place a trapdoor on the upper part of a block (at player height) so that it will close downwards
• Stand directly against the block the trapdoor is placed on
• Activate the trapdoor