Halfway Already?

I personally have not had the situation where a lesson plan that I created for a kid at CAPES! was not used for that kid. BE has been to CAPES! every week. I also have not had a child show up that I have not planned for. So, I don't have any personal experience for having a child show up that we have not written a plan for either.
However, I do believe the most important thing to know and be prepared for in potential situations like this is very simple. If you know your students you should be able to make up a lesson plan on the fly. You need to know what kinds of things your students are working on. What their strengths and weaknesses are. As long as you know your students, know who you are teaching, know your goals with each student, know their goals. If you know your students, really know them then you should be able to come up with a pretty good lesson plan on the fly. You should be able to make accommodations and modifications pretty easily.