Adding mobs to the scoreboard using a command such as:

/scoreboard teams join TeamName @e[type=Creeper]

Adds a new "player" to the team with the matched entites UUID value. On a multiplayer server this prevents additional players from joining the server with the IOException:

Internal Exception: io.netty.handler.codec.DecoderException: java.io.IOException:
The received string length is longer than the maximum allowed (36 > 16)

As the packet is only meant to accept player names as values (which are a maximum of 16 characters long). This issue does not appear to disconnect players who are already on the server however.