For example, last December, Judge Schack tossed out a foreclosure lawsuit filed by U.S. Bancorp after determining that a bank employee, Kim Stewart, had identified herself in two conflicting ways in documents throughout the lawsuit.

In 2008, Ms. Stewart signed an assignment of mortgage — which gives the mortgage servicer the right to foreclose — to U.S. Bancorp, identifying herself as assistant secretary of Mortgage Electronic Registration Systems. Yet in 2009, Ms. Stewart signed a separate document in the lawsuit as vice president of U.S. Bancorp, court records show.

The judge, in a derisive tone, suggested that perhaps the bank and its law firm “do not want the court to confront the conflicted Ms. Stewart,” according to a transcript. U.S. Bancorp strongly disagreed with the judge’s ruling and planned to appeal the decision, said Teri Charest, a spokeswoman for the bank. She added that Ms. Stewart was an officer of the bank and had “signed all documents appropriately.”

George Babcock, a lawyer in Pawtucket, R.I., who represents homeowners, estimated that roughly 300 of his clients were being threatened with foreclosures that included documents signed by Ms. Stewart.

A similar problem has cropped up on the West Coast, where an employee of a mortgage servicing firm whose signature appeared in a lawsuit filed by one of the eight firms had already been flagged as problematic.

Phillip Bennett, a retired schoolteacher in California, was evicted last month from the home he shared with his wife in Rancho Cucamonga.

Mr. Bennett said he thought he might be able to save his home, despite falling behind on his loan payments, because the mortgage assignment was signed by a mortgage company employee, Marti Noriega, who was previously involved in a foreclosure that had been halted.