I logged into the game, and appeared below the terrain. It looked like I was in a falling animation, but I wasn't actually falling. I was just stuck below the terrain.
/stuck didn't fix the problem.
I tried to log out, but apparently falling counts as an action, and I couldn't log out. I tried to quit the game, but again, falling is an action, and I couldn't quit. I had to kill the game from the task manager.
Trying to log back in, I got a mutex error, probably because I had to kill the game, and a lock file wasn't deleted, or however you implement mutexes. Something wasn't cleaned up after killing the game.