Google announced of "completng post-mortem on the Datastore outage App Engine experienced two weeks ago on May 25th. A very small percentage of apps (approximately 2%) have been affected by unapplied writes as a result of the outage. "Unapplied writes" are writes that didn't get replicated to secondary Datastore before completing the fail over process.
We've emailed admins of all affected apps for action requred. If your app does have unapplied writes, all of the data has been recovered and reinserted into your apps Datastore as separately labeled entities. We've developed new tools in Datastore to re-integrate unapplied writes and we've also provided a support email address to help you work one-on-one with App Engine team as well," noted Google.
More Info: Unapplied Writes FAQ