diff -r 8f85262fbe8a Doc/library/imaplib.rst
--- a/Doc/library/imaplib.rst	Sun Jul 06 02:24:24 2014 -0400
+++ b/Doc/library/imaplib.rst	Wed Jul 16 17:16:24 2014 -0700
@@ -470,6 +470,16 @@
          M.store(num, '+FLAGS', '\\Deleted')
       M.expunge()
 
+   .. note::
+
+      .. index:: single: Brackets
+
+      Creating flags containing ']' violates the `RFC 3501
+      <http://tools.ietf.org/html/rfc3501#page-81>`_ and the IMAP
+      protocol. However, popular IMAP servers, such as Gmail, accepts flags
+      with brackets like "[test]" despite the violation. Thus, :mod:`imaplib`
+      allows you create and access flags with brackets for backwards
+      compatibility reasons.
 
 .. method:: IMAP4.subscribe(mailbox)