A few days ago I encounted a total website crash during an upgrade from wordpress version to 4.0 to 4.01.
This is my short story.
I had 2 backup plugins installed. Ready Backup and BackupWP. I quickly found out that I only needed one plugin, and due to its ease of use and great features I sticked with Ready backup.
I imediately setup the plugin to fit my needs and started making backups of both database and content.
I went ahead and tried updating my wordpress version. BIG MISTAKE!!
My site crashed down and locked me out.