I have found that being a teacher requires a lot of flexibility, creativity, and the ability to change plans at the drop of a hat. Snow days, snow delays, assemblies, testing programs, computer crashes, internet failures and a host of other things can leave you with a well thought out lesson plan that you can't use any more. I always tell student teachers and new teachers that they have to be able to adapt to these changes quickly and I've tried to give them some practical advice about it. I've also found that technology can be a great thing to have as a backup.
For example, one day the network connection in our section of the building went down (server crashed.) This meant that we could not access our files on the network drive, or even log into a computer. I was planning on using a PowerPoint presentation in class that day...now I had to change that. So, since I have a Palm Centro with Documents to Go on it, I simply called up the PowerPoint file on my Centro, grabbed a marker, and wrote the notes on the whiteboard for the students. Later, I posted the original file to my website so that the students could look at the pictures I had on the file. I tell new teachers to always print out a set of handouts for their PowerPoint presentations so that they can have a backup plan if their computer or projector fails.
Another good backup is for those days where you find out over half of your class will be missing because of an activity or assembly. I don't want to move ahead because too many students will be left behind, so I want to do something educational and about the topic at hand that they can catch up with on their own. The school district has a subscription to Discovery Streaming. On there are thousands of great educational videos, including Discovery TV shows, and lesson plans to go with them. I'll show the students that are in class the video and do an activity with them, and then post the video link on the class website and have the absentee students watch it at home on their own with a different activity to do.
As a teacher, you always have to have a backup plan and be ready to change your plans at any time.