Sometimes, when servlet sessions are enabled, the app gets an error in the like:

com.google.apphosting.api.ApiProxy$CancelledException: The API call datastore_v3.Put() was explicitly cancelled. at com.google.apphosting.utils.runtime.ApiProxyUtils.convertApiError(ApiProxyUtils.java:75) at com.google.apphosting.utils.runtime.ApiProxyUtils.getApiError(ApiProxyUtils.java:202) at com.google.apphosting.runtime.ApiProxyImpl$AsyncApiFuture.success(ApiProxyImpl.java:691) at com.google.apphosting.runtime.ApiProxyImpl$AsyncApiFuture.success(ApiProxyImpl.java:588) at com.google.apphosting.runtime.http.HttpApiHostClient.receivedResponse(HttpApiHostClient.java:296) at com.google.apphosting.runtime.http.JettyHttpApiHostClient$Listener.onComplete(JettyHttpApiHostClient.java:206) at

It seems related to a possible flake in the API server backend, having a race condition between the close statement of the HTTP request and the API request, and when the API is really called, the API ticket is null, and it should not be null.