Mike Brock had just begun the first bellwether trial over General Motors Co.’s ignition-switch defect when a call came into the automaker’s hotline. A real estate agent, who had heard about the New York trial on the radio, wanted GM’s legal team to know that the plaintiff, Robert Scheuer, had once given him a fabricated $440,000 check toward the purchase of a house.

Brock, a partner in Kirkland & Ellis’s Washington office, wanted to use the information to challenge the plaintiff’s credibility. So on cross-examination, he confronted Scheuer about the check.