Hello
When you place a command block a set the command to /say @e
(or anything else what will print @e to the players/to a player)
and the command block activated, all the connected players will kicked from the server
I think this is because the message is too long.
Is this still an issue in the current Minecraft Snapshot 15w45a or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.