For most of the history of this clause, there was not much reason to challenge its meaning or applicability in federal courts. But in 1965, Pointer v. Texas applied the Confrontation Clause to state courts, which gave it a much wider applicability.
One continuing controversy has been whether “hearsay evidence” is permissible in criminal trials. If a witness claims in court that he heard a third party say something incriminating about the defendant—and the third party is not (and perhaps cannot be) called to the witness stand to be cross-examined by the defense—does that violate the 6th Amendment right to be confronted by one’s accusers? What if the third party gave a credible first-person account prior to the trial but is no longer available to testify? For instance, the witness may have since died, or now refuses to testify on grounds of self-incrimination. A 1980 case, Ohio v. Roberts, had concluded that hearsay evidence could be admissible if it was deemed reliable. But in a 2004 case, Crawford v. Washington, the Court overturned Roberts, deciding unanimously that hearsay testimony (in this case, a tape-recorded statement to the police) may not be admitted.
The courts will sometimes make exceptions to the Confrontation Clause under extraordinary circumstances. For instance, in Maryland v. Craig (1990), a child witness was allowed to testify through closed-circuit television because testifying in court would have been too traumatic for the child. In this case, the Court determined that all of the criteria which make the Confrontation Clause valuable to the defendant were met: the jury was able to view the “manner of delivery” as the child testified, and the defense was able to cross-examine the witness. The courts will grant some exceptions, such as this one, but they must be for good cause and they must not compromise the protections intended by the 6th Amendment.