diff --git a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/AssetLoader.java b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/AssetLoader.java
index e6f122591e..525eaee102 100644
--- a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/AssetLoader.java
+++ b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/AssetLoader.java
@@ -64,17 +64,21 @@ public interface AssetLoader {
         EditedMediaItem editedMediaItem, Looper looper, Listener listener);
   }
 
-  /**
-   * A listener of {@link AssetLoader} events.
-   *
-   * <p>This listener can be called from any thread.
-   */
+  /** A listener of {@link AssetLoader} events. */
   interface Listener {
 
-    /** Called when the duration of the input media is known. */
+    /**
+     * Called when the duration of the input media is known.
+     *
+     * <p>Can be called from any thread.
+     */
     void onDurationUs(long durationUs);
 
-    /** Called when the number of tracks output by the asset loader is known. */
+    /**
+     * Called when the number of tracks output by the asset loader is known.
+     *
+     * <p>Can be called from any thread.
+     */
     void onTrackCount(@IntRange(from = 1) int trackCount);
 
     /**
@@ -85,6 +89,10 @@ public interface AssetLoader {
      *
      * <p>Must be called once per {@linkplain #onTrackCount(int) declared} track.
      *
+     * <p>Must be called from the thread that will be used to call the returned {@link
+     * SampleConsumer}'s methods. This thread is generally different from the one used to access the
+     * {@link AssetLoader} methods.
+     *
      * @param format The {@link Format} of the input media (prior to video slow motion flattening or
      *     to decoding).
      * @param supportedOutputTypes The output {@linkplain SupportedOutputTypes types} supported by
@@ -107,6 +115,8 @@ public interface AssetLoader {
     /**
      * Called if an error occurs in the asset loader. In this case, the asset loader will be
      * {@linkplain #release() released} automatically.
+     *
+     * <p>Can be called from any thread.
      */
     void onError(ExportException exportException);
   }
diff --git a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/EncodedSamplePipeline.java b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/EncodedSamplePipeline.java
index 97543a2a38..c9b010c2cd 100644
--- a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/EncodedSamplePipeline.java
+++ b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/EncodedSamplePipeline.java
@@ -37,7 +37,8 @@ import java.util.concurrent.atomic.AtomicLong;
   private final Queue<DecoderInputBuffer> availableInputBuffers;
   private final Queue<DecoderInputBuffer> pendingInputBuffers;
 
-  private volatile long mediaItemOffsetUs;
+  private long mediaItemOffsetUs;
+
   private volatile boolean inputEnded;
 
   public EncodedSamplePipeline(
diff --git a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/OnMediaItemChangedListener.java b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/OnMediaItemChangedListener.java
index 8ace9765f0..25e4c8d0a8 100644
--- a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/OnMediaItemChangedListener.java
+++ b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/OnMediaItemChangedListener.java
@@ -26,8 +26,6 @@ import com.google.android.exoplayer2.MediaItem;
    * Called when the {@link MediaItem} whose samples are passed to the {@link SamplePipeline}
    * changes.
    *
-   * <p>Can be called from any thread.
-   *
    * @param editedMediaItem The {@link MediaItem} with the transformations to apply to it.
    * @param durationUs The duration of the {@link MediaItem}, in microseconds.
    * @param trackFormat The {@link Format} of the {@link MediaItem} track corresponding to the
diff --git a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/SampleConsumer.java b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/SampleConsumer.java
index 8b82e86079..63ff140930 100644
--- a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/SampleConsumer.java
+++ b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/SampleConsumer.java
@@ -21,11 +21,7 @@ import androidx.annotation.Nullable;
 import com.google.android.exoplayer2.decoder.DecoderInputBuffer;
 import com.google.android.exoplayer2.video.ColorInfo;
 
-/**
- * Consumer of encoded media samples, raw audio or raw video frames.
- *
- * <p>All the methods in this class can be called from any thread.
- */
+/** Consumer of encoded media samples, raw audio or raw video frames. */
 public interface SampleConsumer {
 
   /**
diff --git a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/SamplePipeline.java b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/SamplePipeline.java
index b8372b4549..16321667a0 100644
--- a/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/SamplePipeline.java
+++ b/library/transformer/src/main/java/com/google/android/exoplayer2/transformer/SamplePipeline.java
@@ -36,6 +36,11 @@ import java.util.List;
  * Pipeline for processing media data.
  *
  * <p>This pipeline can be used to implement transformations of audio or video samples.
+ *
+ * <p>The {@link SampleConsumer} and {@link OnMediaItemChangedListener} methods must be called from
+ * the same thread. This thread can change when the {@link
+ * OnMediaItemChangedListener#onMediaItemChanged(EditedMediaItem, Format, long) MediaItem} changes,
+ * and can be different from the thread used to call the other {@code SamplePipeline} methods.
  */
 /* package */ abstract class SamplePipeline implements SampleConsumer, OnMediaItemChangedListener {