Google says it "temporarily lost part of [its] Search index." Essentially, what happened was that the company was deploying a new version of its Search index to all of its data centers across the world. Normally, this is a routine procedure for the company, one that it does frequently to ensure that the data center that's geographically the closest to a user can deliver them the most relevant search results quickly. Partway through that process, however, the search giant says a "small number" of documents were "dropped" (that is, they weren't in the updated index), which is what led to Google Search delisting hundreds of websites. The result for affected websites was lost traffic and revenue.
As a byproduct of the botched deployment, Google's Search Console, a tool the company provides to individuals and companies to help them gauge how their website performs on Search, started showing "inconsistencies." For most users, what that they saw their Search metrics statistics flatline through the variety of reports Google offers as a part of the Search Console.
Google claims its on-call staff caught wind of the issue quickly on the day that it happened, April 7th. However, due to the scale of the company's data center infrastructure, it took until April 11th to roll every data center back to an earlier Search index. It then took almost an additional two extra weeks for Google to fix the Search Console.
Moving forward, the company says it plans to more frequently and promptly to share any relevant information to customers, both through social media as well as through the Search Console itself; in fact, Google reports it put its new communications strategy to the test when a smaller bug affected Search on May 22nd.