When getting into a boat travelling to the nether, after you enter the nether you are forced out of the boat.

To recreate this:

Place a boat into the nether portal and quickly mount it. The boat can be a little hard to mount because of how quickly it will enter the nether. If you mounted the boat, you will end up in the nether and you will no longer be on the boat, even though the screen still gives the prompt to dismount the boat.