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A Chinese developer just caught an interesting record in its server log, saying that a mysterious running Windows Phone OS version 8.10.12166 has downloaded his app.



From WPCentral:
Considering the latest version of Windows Phone that we consumers have direct access to is 8.0.10328.78, the higher version in question has to be Windows Phone 8.1 being tested somewhere inside Microsoft.
  Microsoft testing Windows Phone 8.1 internally