Here is what I do to avoid this happening. When you first open the kit it's a good idea to inventory all of the parts and wood to make sure you have everything you should. As I inventory I use a pencil to label all the wood. Use a ruler on the pieces that you're not quite sure of. As you build and cut off of the sticks cut on the end opposite of where you made your mark. If you take the time to do this before you start building you won't mistake a piece of wood while you are building.