The 802.1q VLAN protocol allows remote attackers to bypass network segmentation and spoof VLAN traffic via a message with two 802.1q tags, which causes the second tag to be redirected from a downstream switch after the first tag has been stripped, as demonstrated by Yersinia, aka "double-tagging VLAN jumping attack."
http://www.securityfocus.com/archive/1/419834/100/0/threaded
http://www.securityfocus.com/archive/1/419831/100/0/threaded
http://lists.grok.org.uk/pipermail/full-disclosure/2005-December/040333.html