This is a bug that causes the scoreboard to mis-behave in a mostly irrecoverable way (I ended up deleting scoreboard.dat to fix it). I have a guess as to the cause and how to reproduce it.

Rather than try to describe the bug, it may be better to just view the video.
http://www.twitch.tv/lorgon/b/536304014
At time code 3:30:10, what happens is, Zhuria's internet glitches, and then things go wonky on the scoreboard in our mini-game. Here is, I believe, the sequence of events:

Zhuria's internet causes her client to disconnect.
Zhuria's internet returns, and she reconnect to the server.
Server says 'Zhuria joined the game'. (But she has not left yet!)
Zhuria sees a duplicate of herself on the server.
Server presumably still had her old connection open, which times out or something. 'Zhuria has left the game' in the chat (but she is still here).
Mini-game begins. It does an @a to popular scoreboard sidebar, and both 'Zhuria' and her GUID appear in the scoreboard.
Later I attempt to 'scoreboard players reset thatGUID', and instead Zhuria herself is removed from the scoreboard. Nothing, not even 'scoreboard players reset *' can remove the GUID from the scoreboard.

I believe this is a symptom of a 'duplicate login'. I think that

• client connects
• client rudely disconnects
• (server does not notice client is gone yet)
• client reconnects

will cause 'X joined the server' to appear twice before the first 'X left the server' appears, and once this happens, some GUID tables where things are supposed to be unique keys become non-unique and then all heck breaks loose on the scoreboard.

The bug, then, is in the server protocol for dealing with the same client trying to log in twice (and inherent race conditions between client and server state on rude disconnects).