diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -648,9 +648,15 @@
 
 .. data:: modules
 
-   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.
+   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.
 
+   .. impl-detail::
+      Re-binding this dictionary to another value will result in undefined
+      behavior and should not be done.  Other builtin modules directly use the
+      interpreter's modules dict, to which sys.modules is initially bound.  So
+      a new value will not receive changes made by those modules.
 
 .. data:: path