What happened in the courtroom that proves that Mr. Carton was much more observant than he pretended to be?

Charles Darnay is on trial in an English court, charged with the very serious crime of treason. It's a completely false charge, but as a Frenchman in England during the Revolution, he's a subject of suspicion and paranoia. Sydney Carton's at the Old Bailey during Darnay's trial, acting as an assistant to the defense counsel, Mr. Stryver. He seems completely bored by the proceedings, spending most of the time staring blankly at the ceiling. However, not for the first time in the story, Sydney surprises us. He demonstrates that he really does have extraordinary powers of observation after all. He draws the attention of Mr. Stryver to the remarkable physical resemblance between himself and the prisoner in the dock. Carton's quick thinking allows Stryver to undermine the credibility of a witness for the prosecution, who can't be sure that Darnay really was the man he saw. As a result, more than reasonable doubt is sown in the jury's mind, and Darnay is acquitted.