Changes made according to comments:

* Creating handleFlowError method
* Using mocks instead of unnecessary spies

amazon-kinesis-client/src/main/java/software/amazon/kinesis/retrieval/fanout/FanOutRecordsPublisher.java

@@ -126,17 +126,7 @@ public class FanOutRecordsPublisher implements RecordsPublisher {
                 outstandingRequests = 0;
 
                 try {
-                    if (t.getCause() instanceof ResourceNotFoundException) {
+                    handleFlowError(t);
-                        log.warn(
-                                "{}: Could not call SubscribeToShard successfully because shard no longer exists. Marking shard for completion.",
-                                shardId);
-                        subscriber.onNext(ProcessRecordsInput.builder().records(Collections.emptyList())
-                                .isAtShardEnd(true).build());
-                        subscriber.onComplete();
-                    } else {
-                        subscriber.onError(t);
-                    }
-
                 } catch (Throwable innerThrowable) {
                     log.warn("{}: Exception while calling subscriber.onError", shardId, innerThrowable);
                 }
@@ -154,6 +144,19 @@ public class FanOutRecordsPublisher implements RecordsPublisher {
         }
     }
 
+    private void handleFlowError(Throwable t) {
+        if (t.getCause() instanceof ResourceNotFoundException) {
+            log.debug(
+                    "{}: Could not call SubscribeToShard successfully because shard no longer exists. Marking shard for completion.",
+                    shardId);
+            subscriber
+                    .onNext(ProcessRecordsInput.builder().records(Collections.emptyList()).isAtShardEnd(true).build());
+            subscriber.onComplete();
+        } else {
+            subscriber.onError(t);
+        }
+    }
+
     private String throwableCategory(Throwable t) {
         Throwable current = t;
         StringBuilder builder = new StringBuilder();

amazon-kinesis-client/src/test/java/software/amazon/kinesis/retrieval/fanout/FanOutRecordsPublisherTest.java

@@ -5,8 +5,8 @@ import static org.junit.Assert.assertThat;
 import static org.junit.Assert.fail;
 import static org.mockito.Matchers.any;
 import static org.mockito.Mockito.doNothing;
+import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
-import static org.mockito.Mockito.spy;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 
@@ -201,23 +201,7 @@ public class FanOutRecordsPublisherTest {
                 .forClass(FanOutRecordsPublisher.RecordFlow.class);
         ArgumentCaptor<ProcessRecordsInput> inputCaptor = ArgumentCaptor.forClass(ProcessRecordsInput.class);
 
-        Subscriber<ProcessRecordsInput> subscriber = spy(new Subscriber<ProcessRecordsInput>() {
+        Subscriber<ProcessRecordsInput> subscriber = mock(Subscriber.class);
-            @Override
-            public void onSubscribe(final Subscription subscription) {
-            }
-
-            @Override
-            public void onNext(final ProcessRecordsInput processRecordsInput) {
-            }
-
-            @Override
-            public void onError(final Throwable throwable) {
-            }
-
-            @Override
-            public void onComplete() {
-            }
-        });
 
         source.subscribe(subscriber);