We study student understanding of the use of a tree structure in the context of an introductory web development course. In particular, we analyze student answers as they use a tree structure to construct file references in web pages. More fundamentally, our study initiates a bottom-up study of computational thinking by identifying the computational thinking mistakes that students make when they are learning resource referencing for web development. Our preliminary results suggest that students do not necessarily learn abstract concepts (like trees) and abstract rules of reasoning (composing relative and absolute tree paths) by just working with folders and composing file references alone.