Quorum Disk configuration.
Last night, I was trying to solve a cluster question. The question was easy to solve but user didn't mention the exact situation and current configuration of production environment and cluster. I was completely failed to help the user. At last, I saw the user didn't mention the quorum configuration. I asked him about the configuration of quorum disk. He replied saying that Quorum disk is configured locally and the node which was holding the disk failed. Hence, other nodes also failed.
Microsoft has published an article for Quorum configuration:
http://support.microsoft.com/kb/280345