Explanation of Gmail's outage

Gmail had a major outage yesterday morning and here's Google's official explanation.

This morning, there was a routine maintenance event in one of our European data centers. This typically causes no disruption because accounts are simply served out of another data center.

Unexpected side effects of some new code that tries to keep data geographically close to its owner caused another data center in Europe to become overloaded, and that caused cascading problems from one data center to another. It took us about an hour to get it all back under control.