Disasters can strike at any time. This is why they are also known as emergencies. While it would be wonderful if we could schedule emergencies, this is certainly not the case. They never take a day off and always seem to strike at the worst possible times. This is why you always need to have a plan for when they do inevitably strike.
For a perfect example, I need to look no farther than myself. Christmas night I had returned home from visiting with family and was just starting to settle in to sleep when there was a knock on my door. I opened it to see my downstairs neighbor. After exchanging pleasantries, he asked me downstairs and into the bathroom which sits right below mine. Up in the corner was a large spot of water damage with a slight sheen along the wall. This was a surprise because there was absolutely no water anywhere in my bathroom that was visible. After some discussion, it turns out that this had been occurring for a couple of days. Well, after a call to the landlord was put in our building was filled with people trying to determine the source of this leak. Like I said, even on Christmas emergencies don't hold off.
Although it is impossible to prevent every kind of emergency, there is no reason to not have a contingency plan for when these types of things occur. Granted it can be difficult to immediately put these plans into motion depending on when the disaster strikes, such as Christmas night. Getting the situation dealt with as quickly as possible, however, should always be one of your top priorities. After you are certain that everyone is safe, trying to mitigate the disaster is normally an extremely difficult task.
Some disasters are simply too bizarre to try and plan for. It is not humanly possible to map out each and every single thing that could possibly go wrong so if something does occur that you don't have a plan for, don't panic. The only thing you can do is roll with what you're given. Ask around to see what other landlords and property owners have dealt with and see if you can add to your list of plans. We can only hope to be ready for when these disasters do strike.