This article describes how a free, web-based intelligent tutoring system, (ASSISTment), was used to create online error analysis items for preservice elementary and secondary mathematics teachers. The online error analysis items challenged preservice teachers to analyze, diagnose, and provide targeted instructional remediation intended to help mock students overcome common error patterns and misconceptions. A short description of how the ASSISTment system was used to support follow-up in-class discussions among preservice teachers is provided, as well as suggestions for producing similar online error analysis items in other content areas. Directions for accessing all of the mathematics error analysis problem sets currently available in the ASSISTment system, sample error analysis items and responses, and a rubric for implementing these assignments in mathematics methods classes to support preservice teachers are included at the conclusion of the article.