Sometimes bounding past Wall Street expectations isn't enough. On Wednesday, investors fled shares of Doral Financial ( DRL), even though the bank's fourth-quarter earnings of $1.22 a share exceeded the Thomson Financial consensus estimate by 18 cents. Investors were spooked by the potentially low quality of the bank's earnings and the possibility that it is sitting on a derivatives mess. In early afternoon trading, shares of the Puerto Rico-based mortgage lender were down $5.48, or 11%, to $44. At 2 p.m. EST, more than 6 million shares had changed hands, 15 times the stock's normal trading volume. The selloff may mark an end of a huge run for tiny Doral. Before today, shares of Doral were up 170% since January 2003, benefiting from the bank's small but lucrative role as a real estate financier in Puerto Rico. On the surface, the numbers released late Tuesday looked great. Doral reported earning $150.5 million, or $1.22 a share, compared with $94.7 million, or 76 cents a share, in the year-ago period. But on closer inspection, Doral's big earnings beat looks a bit hollow. That's because the bank's bottom line was juiced by a $77 million tax benefit stemming from a temporary 50% reduction in Puerto Rico's long-term capital gains rate. The tax benefit applies to transactions between July 1, 2004, and June 30, 2005. Doral claims the tax reduction merely offset a $95 million trading loss it incurred on some investment securities the bank uses as part of its strategy to hedge against interest rate fluctuations. The bank says the new law prompted it to "accelerate" the time frame for recording an impairment charge on the value of those securities called interest-only strips. The bank recorded a $97.5 million pretax impairment charge on the securities as the result of an increase in interest rates, specifically a rise in LIBOR -- the London interbank offered rate, which is frequently used as a rate benchmark.