Mathematics is a core academic subject, regardless of grade level or setting. Everyone uses mathematics in their everyday life, so being competent in basic mathematics is critical to independent living. One thing teachers can do to ensure learners are learning the mathematics concepts being taught is to diagnose and remediate the errors they are seeing. This skill involves digging deeper into the work of the learners and looking for error patterns. Unfortunately, this skill is not a focus of mathematics instruction courses that pre-service teachers (PSTs) take in their program. This article is aimed at mathematics instruction faculty and describes three main error types as well as what to do when they are identified.