Server hang on startup

[devryb]

Had to rollback the nether on our world to resolve this.
Nothing shown in logs or crash report generated

http://puu.sh/kSTGk/3c310d2fba.png

[devryb]

Thanks to the help with the player involved on this we found what causes this.
If you place a quarry plus in the nether high enough that it attempts to break the bedrock ceiling it will cause this hang.