Every time when a player leaves the server it outputs the message to the log with the whole TextComponent object rather than just its Text-property, for example:
[19:23:55] [Server thread/INFO]: Notch lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
This makes the log harder to read afterwards. It's only a minor annoyance though.

I'd rather expect:
[19:23:55] [Server thread/INFO]: Notch lost connection: Disconnected

Code analysis by marcono1234 can be found in this comment.