Few years back we had nation-wide panic that Parliament election results were unavailable for hours after closing polls due to “IT issue”. Analysis showed that everything was right, load tested, cached etc, except real life situation of added few million rows and few thousands queries a second late. Automatic query planner just failed and all the smartest experts in country could do was to wait until it self-heals. Imagine national TV screening for hours “no results available” and You giving updates to the PM every 5 minutes. And all you know that your most important DB in country cannot figure it out... True story