If you're experiencing unexplained site behavior, in my case I was getting the "a critical error has occurred on your site" white screen on only one specific instance of a deployed intranet stack with all other conditions the same to working deployments (system config, plugin versions, etc.). In the logs there were no critical errors only intermittent memory exhausted errors. I upped the wp_memory_limit to 96M (previously 32M) and rebooted the server. Everything seemed to resolve. Moral of the story is I spent 60+ min activating/deactivating plugins, cross-referencing system configurations, blah, blah, blah, when if I'd stuck to basic diagnostics (check logs and make common sense actions) I would have resolved the site's issue much more quickly.