If you dismount a boat, and there is a wall in front of you (such that you cannot dismount directly in front and must instead be dismounted on top of the boat), and the boat is in water, you will get stuck inside of the boat and will be unable to move until the boat is broken.

This does not happen while flying, nor while the boat is on land. It also is slightly inconsistent and may take a few tries to occur.

To reproduce

1. Build a setup where a boat is in water and there is a wall that extends at least 2 blocks above the water directly in front of the boat (see 2016-11-11_15.57.28.png)
2. Get in the boat
3. Press shift to dismount
4. You should be stuck inside of the boat; if not, break and replace the boat and try again