Minecraft wiki entry for Command_Block, associated text: Command blocks execute commands with OP level 2. The following commands cannot be used in a command block: /ban, /banlist, /ban-ip, /debug, /deop, /kick‌[JE only], /op, /pardon, /pardon-ip, /publish, /reload‌[BE only], /save-all, /save-off, /save-on, /stop, and /whitelist (i.e., /debug, /publish, and all of the multiplayer-only commands except /list).