This has never happened to me before, so I don't know how to reproduce it, but I think I know how to. (Other players may be needed?)
So, I logged onto the server, and I kept glitching halfway into a block (Also a bug. You should make players spawn in the nearest two-block-high or more area above the player if he spawns in a block). So anyways, on that server, you need to login with a command, but the chat kept closing after opening it so I didnt have time to type the login thing and get out of the block. Then, when I was going to exit, the exit menu also closed, so I just did F3+Crash. I don't know if it will help or not, but I hope it does.