This document summarizes common investment mistakes made by individual investors based on an analysis of 250,000 portfolios. It finds that over 90% of investors have problems like poor diversification, high fees, and neglecting their portfolios. Specifically, 9 in 10 investors have poor diversification by concentrating their holdings in single stocks or asset classes. 6 in 10 pay too high fees, such as expense ratios over 0.5% or paying advisors 1.3% of assets annually. 1 in 4 portfolios show signs of cash drag, neglect, or overtrading.