Index: model-api.txt
===================================================================
--- model-api.txt	(revision 2360)
+++ model-api.txt	(working copy)
@@ -170,6 +170,14 @@
     ``primary_key=True`` implies ``blank=False``, ``null=False`` and
     ``unique=True``. Only one primary key is allowed on an object.
 
+    .. note::
+    	The primary key value is used to construct the (relative) URL for
+	editing the object in the admin interface. If you primary key turns
+	out to be a well-formed URL (such as ``http://....`` or
+	``mailto:...``), this can lead to not being able to edit the object.
+	So do not set ``primary_key = True`` on fields that can take fully
+	qualified URLs as values.
+
 ``radio_admin``
     By default, Django's admin uses a select-box interface (<select>) for
     fields that are ``ForeignKey`` or have ``choices`` set. If ``radio_admin``