An analysis of the 1990 National Assessment of Educational Progress (NAEP) found that only 46 percent of all high school seniors demonstrated success with a grasp of decimals, percentages, fractions and simple algebra. This article investigates error patterns that emerge as students attempt to answer questions involving the ability to apply fraction concepts and perform operations on fractions, providing a source to assist teachers in detecting and correcting common mistakes students make when manipulating fractional numbers. The analysis is based on a twenty-five question test covering five categories: (1) Algorithmic applications; (2) Applications of basic fraction concepts in word problems; (3) Elementary algebraic concepts; (4) Specific arithmetic skills that are a prerequisite to algebra; and (5) Comprehension of the structure of rational numbers. Each test question is briefly analyzed and examples of both common and unique errors are included. Implications for practice are discussed and recommendations proposed.