A federal prosecutor thought she might have caught Cliff Sloan in a lie Tuesday—and that she had the proof in hand.

U.S. District Judge Amy Berman Jackson of the District of Columbia had warned Greg Craig several days earlier to not interact with his former law firm colleagues as he stood trial in Washington federal court on charges that he misled the Justice Department about his work for Ukraine while he was a partner at Skadden, Arps, Slate, Meagher & Flom. The judge’s admonition underscored an awkward element of Craig’s trial: His friends and former colleagues have been called to testify against him.