When connecting OVS to ODL - if ports already exist that ODL would want to create, it will fail to reconcile and result in errors. This was seen in Boron. For example, if vxlan tunnel ports already exist, I see ODL create 2 more vxlan ports, instead of using the ones that already exist:

[root@overcloud-novacompute-1 ~]# ovs-vsctl show
be5e9b0f-2a00-4b71-beae-2279c1826525
Manager "tcp:192.0.2.4:6640"
is_connected: true
Bridge br-int
Controller "tcp:192.0.2.4:6653"
is_connected: true
Controller None
fail_mode: secure
Port "tap4a993a0d-12"
Interface "tap4a993a0d-12"
Port "tune12ce5d9e46"
Interface "tune12ce5d9e46"
type: vxlan
options:

{key=flow, local_ip="11.0.0.26", remote_ip="11.0.0.21"}
Port "tun55eac6c4536"
Interface "tun55eac6c4536"
type: vxlan
options: {key=flow, local_ip="11.0.0.26", remote_ip="11.0.0.27"}
Port "tune230956df1c"
Interface "tune230956df1c"
type: vxlan
options: {key=flow, local_ip="11.0.0.26", remote_ip="11.0.0.21"}

Port "tapdf0e53b9-59"
Interface "tapdf0e53b9-59"
Port "tunad8086e8030"
Interface "tunad8086e8030"
type: vxlan
options:

{key=flow, local_ip="11.0.0.26", remote_ip="11.0.0.21"}

In another example, if I am using br-ex as my external network provider mapping. ODL usually will come up and create a patch port between br-ex and br-int. However, if that patch port already exists - ODL will error (in netvirt):
2017-01-30 13:50:19,051 | ERROR | pool-47-thread-1 | NatInterfaceStateChangeListener | 326 - org.opendaylight.netvirt.natservice-impl - 0.3.3.Boron-SR3 | Unable to process add for interface 192152954067022:br-ex-patch ,since Interface ConfigDS entry absent for the same
2017-01-30 14:15:49,096 | ERROR | pool-46-thread-1 | SubnetOpDpnManager | 317 - org.opendaylight.netvirt.vpnmanager-impl - 0.3.3.Boron-SR3 | Cannot get, portOp for port 192152954067022:br-ex-patch:flat is not available in datastore