diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index ec12e02fb37d45..8b3c6fd7627317 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -1073,7 +1073,11 @@ always available.
    This is a dictionary that maps module names to modules which have already been
    loaded.  This can be manipulated to force reloading of modules and other tricks.
    However, replacing the dictionary will not necessarily work as expected and
-   deleting essential items from the dictionary may cause Python to fail.
+   deleting essential items from the dictionary may cause Python to fail.  If
+   you want to iterate over this global dictionary always use
+   ``sys.modules.copy()`` or ``tuple(sys.modules)`` to avoid exceptions as its
+   size may change during iteration as a side effect of code or activity in
+   other threads.
 
 
 .. data:: orig_argv