A class action can have it all—thousands of potential members, a common grievance, solid lead plaintiffs, highly skilled counsel—and still fall apart if the expert witness can’t deliver.

That’s what happened when a federal judge in San Antonio this week sided with lawyers from Norton Rose Fulbright; Gibson, Dunn & Crutcher and Simpson Thacher & Bartlett and refused to certify an 11,000-member class of registered nurses who alleged that local hospitals conspired to fix their wages.