Adobe eBook Reader allows a user to bypass restrictions for copy, print, lend, and give operations by backing up key data files, performing the operations, and restoring the original data files.
http://www.kb.cert.org/vuls/id/438867
http://www.iss.net/security_center/static/9634.php
http://lists.grok.org.uk/pipermail/full-disclosure/2002-July/000177.html