Last week found a race condition during VLAN creation, reported as LP#785890.
You could see it when starting lots of instances in one project after fresh reboot / new installation.
Another bug which we found is inappropriate defaults in dnsmasq which prevents having more than 150 instances per network. Reported as LP#785763.
Both bugs are fixed now and available in trunk.
Good news are that we also backported them to our Cactus build. Enjoy!