Swim wrong height
When swimming in the water, you see the swimming animation, but the character maintains a height of 2 blocks, and not a single block as it should be according to the animation. It is impossible to go through a hole in a block while swimming.
What minecraft version are you on? This does not occur for me.
Also what version is the server you are playing on? Do you use Multiconnect?
EDIT: It's been determined that this is an issue with AntiHunger specifically