diff -r d58c63ff5bb2 Doc/library/json.rst
--- a/Doc/library/json.rst	Wed Apr 13 05:37:29 2011 +0300
+++ b/Doc/library/json.rst	Wed Apr 13 11:21:48 2011 +0300
@@ -156,6 +156,11 @@
    :meth:`default` method to serialize additional types), specify it with the
    *cls* kwarg; otherwise :class:`JSONEncoder` is used.
 
+   .. note::
+
+      Unlike :mod:`pickle` and :mod:`marshal`, JSON is not a framed protocol so
+      trying to serialize more objects with repeated calls to :func:`dump` and
+      the same *fp* will result in an invalid JSON file.
 
 .. function:: dumps(obj[, skipkeys[, ensure_ascii[, check_circular[, allow_nan[, cls[, indent[, separators[, encoding[, default[, **kw]]]]]]]]]])