diff --git a/testing/insulating_clients.rst b/testing/insulating_clients.rst
index dd4362e7e47..65a46007f38 100644
--- a/testing/insulating_clients.rst
+++ b/testing/insulating_clients.rst
@@ -43,3 +43,11 @@ clean PHP process, thus avoiding any side-effects.
 
     As an insulated client is slower, you can keep one client in the main
     process, and insulate the other ones.
+
+.. caution::
+
+    Insulating tests requires some serializing and unserializing operations. If
+    your test includes data that can't be serialized, such as file streams when
+    using the ``UploadedFile`` class, you'll see an exception about
+    *"serialization is not allowed"*. This is a technical limitation of PHP, so
+    the only solution is to disable insulation for those tests.