I had previously logged out while swimming in the ocean with a horse attached to a lead.

When I logged back in, I expected to be in the same situation. However, the moment I got back, the chunks around me were not loaded in (chunks farther out had been so I could see a wall of water on all four sides. I myself was very slowly falling downward, suspended over nothing (reminiscent of falling through the world but at a very slow pace).Within seconds, I started taking damage (as did my horses, which I could not see but could hear taking damage). And as a result I died of 'drowning'. I swam back to the area I think I died in and there no longer appears to be a chunk error, but there is still a glitch when it comes to relogging.