Microsoft's search engine Bing was out Thursday between 6:30 p.m. and and 7 p.m. Pacific Time. In a message posted on the Bing Community blog, Satya Nadella, senior vice president of the Online Services division, said the Bing team was doing "some internal testing that had unfortunate and unintended consequences."

Nadella also said Microsoft is running a postmortem to figure out how to prevent it from happening again.

Having the service available 24/7 is key not only to Microsoft's search business, which is trying to gain share against Google, but for Microsoft's reputation as a provider of cloud-computing services. Businesses have to believe that Microsoft can reliably and securely host their applications and data in the Microsoft data centers.

A few months ago, users of the Sidekick mobile device, run by a Microsoft subsidiary, lost their phone data for several weeks, another blow for the Microsoft cloud.

Bob Muglia, president of the Server and Tools Business, highlighted Bing's resiliency at his keynote speech at the Professional Developers Conference in November. Muglia complained that his online banking frequently went down on Sunday mornings for software updates, and said that Microsoft servers on its cloud platform Azure would always be on.

"Bing is the best example of this. It’s a service that’s always available, it’s highly resilient," Muglia said.