Comment 2 for bug 1313796

I can confirm the problem in Ubuntu 14.04.1 server: I configured a bridge for openvpn:

iface br0 inet static
 address 192.168.1.6
 netmask 255.255.255.0
 gateway 192.168.1.1
 bridge_ports eth0 tap0
 dns-nameservers 192.168.1.1
 dns-search mydomain.com
 pre-up openvpn --mktun --dev tap0
 post-down openvpn --rmtun --dev tap0

and for 50% of computer boots (20% on a slower pc) the br0 setup fails and contains only tap0.

After some debugging, here are my guesses: I found that "brctl addbr br0" is called twice: from /lib/bridge-utils/ifupdown.sh and from /lib/udev/bridge-network-interface (the second one via udev due to openvpn --mktun ???).
When ifupdown.sh is called after bridge-network-interface, "brctl addbr br0" fails and returns nonzero, so the script exits and no interface are added to br0.
When the two scripts are called in the reverse order, all seems to work fine.

I used the proposed workaround to put BRIDGE_HOTPLUG=no /etc/default/bridge-utils in and now my system is working.