diff --git a/docs/api-output.md b/docs/api-output.md
index 0f97e86d..e54c943c 100644
--- a/docs/api-output.md
+++ b/docs/api-output.md
@@ -279,6 +279,8 @@ Use tile-based deep zoom (image pyramid) output.
 Set the format and options for tile images via the `toFormat`, `jpeg`, `png` or `webp` functions.
 Use a `.zip` or `.szi` file extension with `toFile` to write to a compressed archive file format.
 
+Warning: multiple sharp instances concurrently producing tile output can expose a possible race condition in some versions of libgsf.
+
 **Parameters**
 
 -   `tile` **[Object][15]?** 
diff --git a/lib/output.js b/lib/output.js
index 0bb880d4..db2df2b2 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -390,6 +390,8 @@ function toFormat (format, options) {
  * Set the format and options for tile images via the `toFormat`, `jpeg`, `png` or `webp` functions.
  * Use a `.zip` or `.szi` file extension with `toFile` to write to a compressed archive file format.
  *
+ * Warning: multiple sharp instances concurrently producing tile output can expose a possible race condition in some versions of libgsf.
+ *
  * @example
  *  sharp('input.tiff')
  *   .png()