On Columbia, the accident wasn't unforseen in any way - they'd seen similar damage previously, just never to such an extent. And they figured the probabilities of such an event were so remote they didn't need addressing.
Murphy and his law caught up.
On airliners and engine fires : not neccesarily common, but it CAN
happen and there are established procedures for such that are practiced often by the crews both in class and in simulators.
And the aircraft themselves are equipped to deal with fires ... you can dump extinguishing agents into the affected engine, cut off the fuel flow (no fuel, no fire), etc etc.
All of the above, if they work correctly, should allow a safe (albeit white knuckled) landing every time.
Very scary, yes. Very dangerous, possibly. A Disaster? not unless a whole host of things that are supposed to happen correctly do not.