diff --git a/Doc/tutorial/datastructures.rst b/Doc/tutorial/datastructures.rst
--- a/Doc/tutorial/datastructures.rst
+++ b/Doc/tutorial/datastructures.rst
@@ -589,6 +589,19 @@
    orange
    pear
 
+It is not safe to modify a sequence being iterated over while in the loop.
+If you need to modify the list you are iterating over (for example, to
+duplicate selected items), you can iterate over a copy and modify the
+original.  The slice notation makes this particularly convenient::
+
+   >>> words = ['cat', 'window', 'defenestrate']
+   >>> for w in words[:]:  # Make a slice copy of the entire list.
+   ...     if len(w) > 6:
+   ...         words.insert(0, w)
+   ...
+   >>> words
+   ['defenestrate', 'cat', 'window', 'defenestrate']
+
 
 .. _tut-conditions: