Planning for the unexpected?

I'm beginning to notice that despite the countless hours put into planning lessons, there will always be things that arise that were not planned. For instance, the fire drill midday, the child who gets sick, or even unforseen behavioral problems.

I'm realizing that lesson plans often must be taken as a suggestion or as a possibility. Allowing oneself to stray from the planned activities creates flexibility and allows for new opportunities...Sure this will be hard to deal with, especially during the lessons that took hours to prepare, but part of being a teacher is learning how to handle anything.

I'm not suggesting that teachers throw away the planner, but I think my cooperating teacher presented me with good advice. He told me that I should always be ready for the unexpected and always have an emergency back-up activity. There will always be new things that we, as teachers, will face daily in the classroom, many of which could not have been taught in a textbook due to the overwhelming wealth of contingencies.