What does it look like to "understand" operations with fractions? The Common Core State Standards for Mathematics (CCSSM) uses the term "understand" when describing expectations for students' knowledge related to each of the fraction operations within grades 4 through 6 (CCSSI 2010). Furthermore, CCSSM elaborates that students should be able to perform fraction computations using visual models and when such problems are presented in context. These questions then arise: What does it mean to "understand fraction operations"? What types of issues occur that might not be encountered otherwise when solving problems in context and using visual models? In this article, the authors share their experiences working with prospective teachers to describe the teachers' errors and how those errors might be addressed.