Buffett and Peter Lynch's investment in Fannie Mae was a highly successful 'compounder' through the 1990s that went up many-fold due to a strong market position (a quasi-duopoly), but Buffett sold in 2001 when he noticed management was making bets outside their core competence on securities underwriting, which was a precursor to what eventually led to the 2008 financial crisis, demonstrating the importance of monitoring how companies allocate capital.