ftpd in linux-ftpd 0.17, and possibly other versions, performs a chdir before setting the UID, which allows local users to bypass intended access restrictions by redirecting their home directory to a restricted directory.
http://www.securityfocus.com/bid/21000
http://www.debian.org/security/2006/dsa-1217
http://security.gentoo.org/glsa/glsa-200611-05.xml
http://secunia.com/advisories/22997
http://lists.grok.org.uk/pipermail/full-disclosure/2006-August/049014.html