I tried to set the firewall state-policy. When committing, I got the following error:
[email protected]# set firewall state-policy established action accept  [email protected]# commit [ firewall state-policy ] ip6tables: Chain already exists. create_ipt_chain [/sbin/ip6tables -t filter -N VYATTA_STATE_POLICY_FWD_HOOK] failed: [error code - 256] [[firewall state-policy]] failed Commit failed
Following some advice on IRC, I did a reboot and the problem had gone away.