Then he tried to pay off a loan on a home his family had bought in Dallas in 1988. The balance was roughly $100,000, and the property was valued at about $175,000, Mr. Lavalle said. But when he combed through figures provided by his lender, Savings of America, he found substantial discrepancies in the accounting that had inflated his bill by $18,000. The loan servicer had repeatedly charged him late fees for payments he had made on time, as well as for unnecessary appraisals and force-placed hazard insurance, he said.

Mr. Lavalle refused to pay. The bank refused to bend. The balance rose as the bank tacked on lawyers’ fees and the loan was deemed delinquent. The fight continued after his mortgage was allegedly sold to EMC, a Bear Stearns unit.

Unlike most people, Mr. Lavalle had the time and money to fight. He persuaded his family to help him pay for a lawsuit against EMC and Bear Stearns. Seven years and a small fortune later, they lost the house in Dallas. Back then, judges weren’t as interested in mortgage practices as some are now, he said.

The experience lit a fire. Mr. Lavalle set out to learn everything he could about the mortgage industry. In a five-hour interview in Naples, Fla., last month, he described his travels nationwide. He dove into mortgage arcana, land records and court filings. By 1996, he had identified what appeared to be forged signatures on foreclosure documents, foreshadowing troubles to come. He took his findings to big players in the industry: Banc One, Bear Stearns, Countrywide Financial, Freddie Mac, JPMorgan, Washington Mutual and others. A few responded but later said his claims were not valid, he said.

Now he splits his time between Orlando and Boca Raton, advising lawyers as an expert witness. “From my own personal experience and 20 years of research and investigation, nothing — and I mean nothing — that a bank, lender, loan servicer or their lawyer says or puts on paper can be trusted and accepted as true,” Mr. Lavalle said.

FANNIE MAE, now in government hands, has acknowledged how abusive foreclosure practices can hurt its own business. “The failure of our servicers or a law firm to apply prudent and effective process controls and to comply with legal and other requirements in the foreclosure process poses operational, reputational and legal risks for us,” it said in a 2010 filing with the Securities and Exchange Commission.

Five years earlier, Fannie seemed to have taken a different view. That was when Mr. Lavalle pointed out legal lapses by some of its representatives. Among them was the law offices of David J. Stern, in Plantation, Fla., which was handling an astonishing 75,000 foreclosure cases a year — more than 200 a day. In 2005, Mr. Lavalle warned Fannie Mae that some judges had ruled that the Stern firm was submitting “sham pleadings.” Nonetheless, Fannie continued to do business with the firm until it closed its doors last year, after evidence emerged of rampant forgeries and fraudulent filings.