OpenBSD 3.4 and NetBSD 1.6 and 1.6.1 allow remote attackers to cause a denial of service (crash) by sending an IPv6 packet with a small MTU to a listening port and then issuing a TCP connect to that port.
https://exchange.xforce.ibmcloud.com/vulnerabilities/15044
http://www.securityfocus.com/bid/9577
http://www.openbsd.org/cgi-bin/cvsweb/src/sys/netinet6/ip6_output.c
http://www.guninski.com/obsdmtu.html
http://marc.info/?l=bugtraq&m=107604603226564&w=2
http://lists.grok.org.uk/pipermail/full-disclosure/2004-February/016704.html