I first encountered this bug when I was trying to install a one command creation on my server. I did some experimentation, and I found out that it was because of command block minecarts.

What I expected to happen was...:
When I summoned a command block minecart or entered a command into one, a command block minecart would spawn or my command would be set.

What actually happened was...:
When I summoned a command block minecart, I was immediately kicked from my server, and when I placed down a command block minecart and tried to enter a command, I was also kicked from my server. I had to go into the console and do /kill @e[type=MinecartCommandBlock].

Steps to Reproduce:
1. Go into a 15w34b multiplayer server.
2. Do /summon MinecartCommandBlock.
3. You will see that you were kicked.
4. Either create a new world or /kill @e[type=MinecartCommandBlock].
5. Do /give @p command_block_minecart
6. Place the command block minecart on a rail and try entering a command into it.
7. If you press "Done", you will see that you have gotten kicked.