Same error with Ipad 3 wifi version. Failed during upgrade and now restore compates but validating restore fails with error 27.
Cannot use device at all as it is stucked on recovery mode. Tried anything.
Apple parnter service did simply charged me for trying but did not know what is wrong with the device. The fix = new one.
This is totally a bug in the SW or the upgrade broke the Ipad. Good job Apple. Now we have to get new one.