This issue did not happen to my minecraft, but to my firends one, so:
When my friend tried to join his private minecraft server (this happens nearly every time he joins), The chat is filled with
"KarateKarotte joined the game,
KarateKarotte left the game,
KarateKarotte left the game,
KarateKarotte left the game,
KarateKarotte joined the game"
in a random order. Actually he is never on the server (you can't see him using tab).
In the Attachment you can see the user created crash report and the server.log file
[There coud be some other failures in the crash file, because of a power failure, but this also happend much earlier]