diff --git a/clients/src/test/java/org/apache/kafka/clients/MockClient.java b/clients/src/test/java/org/apache/kafka/clients/MockClient.java
index 0dd09d2b4d8..0dd42f8fac3 100644
--- a/clients/src/test/java/org/apache/kafka/clients/MockClient.java
+++ b/clients/src/test/java/org/apache/kafka/clients/MockClient.java
@@ -276,7 +276,6 @@ public class MockClient implements KafkaClient {
         maybeAwaitWakeup();
         checkTimeoutOfPendingRequests(now);
 
-        List<ClientResponse> copy = new ArrayList<>(this.responses);
         // We skip metadata updates if all nodes are currently blacked out
         if (metadataUpdater.isUpdateNeeded() && leastLoadedNode(now) != null) {
             MetadataUpdate metadataUpdate = metadataUpdates.poll();
@@ -287,9 +286,11 @@ public class MockClient implements KafkaClient {
             }
         }
 
+        List<ClientResponse> copy = new ArrayList<>();
         ClientResponse response;
         while ((response = this.responses.poll()) != null) {
             response.onComplete();
+            copy.add(response);
         }
 
         return copy;
diff --git a/clients/src/test/java/org/apache/kafka/clients/admin/AdminClientUnitTestEnv.java b/clients/src/test/java/org/apache/kafka/clients/admin/AdminClientUnitTestEnv.java
index 89f5fdec2a5..6023c63022b 100644
--- a/clients/src/test/java/org/apache/kafka/clients/admin/AdminClientUnitTestEnv.java
+++ b/clients/src/test/java/org/apache/kafka/clients/admin/AdminClientUnitTestEnv.java
@@ -35,6 +35,10 @@ import java.util.Map;
  * To use in a test, create an instance and prepare its {@link #kafkaClient() MockClient} with the expected responses
  * for the {@link AdminClient}. Then, use the {@link #adminClient() AdminClient} in the test, which will then use the MockClient
  * and receive the responses you provided.
+ *
+ * Since {@link #kafkaClient() MockClient} is not thread-safe,
+ * users should be wary of calling its methods after the {@link #adminClient() AdminClient} is instantiated.
+ *
  * <p>
  * When finished, be sure to {@link #close() close} the environment object.
  */