While swimming up flowing water(using the new diving/swimming animation), and brushing up against a wall, the player is sometimes teleported into the wall, and takes suffocation damage. If you mine the block, you no longer take suffocation damage and are standing in a 1x1 block. I managed to reproduce this numerous times while exploring underwater caves.
After writing this, I went and replicated the bug again, this time I took pictures:
Here is the setup for the bug:
All you have to do is press CTRL to swim, fight against the current, then press jump at the same time. You will be teleported into the ceiling, or a wall nearby. here are two pictures of being teleported into the ceiling:
This is very easy to replicate, assuming the setup is correct.
Confirmed in Windows 10 using 1.4.2.0.
Requires Survival mode. While sprint-swimming, the player can clip into solid blocks next to or above the water. If you try to jump while swimming, you will briefly become vertical again at the top of the water, but will not exit swimming mode. But under the right conditions (which seems to be at a certain depth of water), you can exit swimming mode while clipping into a solid block, at which point you can begin to suffocate, or you may be teleported above it if there is air above the block.
Steps to reproduce:
Construct a channel of flowing water with a roof of solid blocks 2 spaces above it.
Standing at the shallow end, press Crouch and Forward together and hold to swim upstream.
Press Jump and hold until the water height plus your jump height lifts you high enough to get stuck in the solid block, which makes you exit swimming mode. You may have to try a second time, but it happens more often than not.
Also, if you start the channel with a water source block on top of a solid block, and you don't get stuck, when you get to the source you will exit swimming mode with your head in the source and your feet in the solid block under it.