From a Fourth Circuit opinion yesterday - this should not happen:
At a preliminary hearing before a magistrate judge on May 17, 2005, the government mistakenly produced an individual named Francisco Almaraz Soriano, rather than the defendant, Francisco Soriano-Jarquin. At the time, no one alerted the court to the error. On the stand, ICE Special Agent Jason Fulton identified the individual present as Francisco Soriano-Jarquin, though upon cross-examination Fulton stated that he could not be sure the individual was Soriano-Jarquin. The judge found probable cause to hold Soriano-Jarquin.Not that it kept the guy from being convicted, of course.