Index: optparse.rst
===================================================================
--- optparse.rst	(revision 69992)
+++ optparse.rst	(working copy)
@@ -510,6 +510,14 @@
   default: ``"usage: %prog [options]"``, which is fine if your script doesn't take
   any positional arguments.
 
+  If using the ``%`` string formatting operator to insert computed elements into a
+  usage string, then you will need to unsure that the ``%prog`` is properly escaped, 
+  so that it isn't interpreted as containing an anomalous ``%p`` formatting token. 
+  This escaping is done with a second percent sign, so that the formatted string 
+  contains the ``%prog`` looked for by :mod:`optparse`, e.g.::
+
+     usage = "usage: %%prog ... %s" % mystring
+
 * every option defines a help string, and doesn't worry about line-wrapping---
   :mod:`optparse` takes care of wrapping lines and making the help output look
   good.