diff -r 6f4627a65c0a Doc/library/sys.rst
--- a/Doc/library/sys.rst	Sun May 05 08:14:53 2013 +0200
+++ b/Doc/library/sys.rst	Sat May 18 12:05:00 2013 -0400
@@ -757,6 +757,7 @@
 
    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 it or deleting critical entries may cause Python to fail.
 
 
 .. data:: path
diff -r 6f4627a65c0a Misc/NEWS
--- a/Misc/NEWS	Sun May 05 08:14:53 2013 +0200
+++ b/Misc/NEWS	Sat May 18 12:05:00 2013 -0400
@@ -245,6 +245,10 @@
 
 - Issue #17327: Add PyDict_SetDefault.
 
+Documentation
+-------------
+
+- Issue #17953: Document sys.modules not to be reassigned
 
 IDLE
 ----