Having ongoing openflow clustering design, the event can be missed in the following scenario.

1) Lets say we have 3 controllers connected to a switch (presume handshake happens from 3 controllers). And lets say controller 1 is the master.
2) A port down event sent from switch to controller 1 and controller 1 crashed without handling it.
3) Switchover happens from controller 1 to controller 2
4) When controller 2 takes up the leadership, it wont query for port status and start handling subsequent events. The port down event is missed!
5) UI will show port up while the port is down in network