When this tag is applied to a message, the subject encoding is not properly set. This tag is added in front of encoding specification and so breaks subject.
$undecipherable_subject_tag = 'WARNING: Unchecked encrypted attachment(s) *** ';
Subject: WARNING: Unchecked encrypted attachment(s) ***
=?iso-8859-1?Q?Statement for 2008-10-31?=
Problem is probably visible with other "subject insertion" settings too...