If you create a long fake player name in the scoreboard system, then attempt to show that player's name in the sidebar, you get a java exception:

Internal Exception: io.netty.handler.codec.DecoderException: The received string length is longer than maximum allowed (47 > 40)

Once this occurs, no players are able to connect to that server. (They all receive the same error)

Steps to reproduce:

/scoreboard objectives add Test dummy
/scoreboard players add ThisNameIsLongerThan40CharactersDontYouKnowThat Test 1
/scoreboard objectives setdisplay sidebar Test

The expected scenario is that it should just truncate the name, or not allow you to save a name longer than the maximum.

Work around:
Gain access to the server's console and run the following command:

/scoreboard objectives remove Test