In the case of Delaware v. Fensterer, the U.S. Supreme Court ruled in 1985 that a defendant's Sixth Amendment right to confront witnesses was not violated when an expert witness could not recall the specific basis for his or her opinion during cross-examination. The court held that as long as a defendant has an opportunity to cross-examine a witness, it does not matter if the witness cannot remember certain details about their testimony. The decision came after Herbert Fensterer was convicted of rape and kidnapping in Delaware based on hair analysis evidence presented by an expert who couldn't recall which microscopic characteristics led him to conclude that hairs found at crime scene were similar to those of Fensterer’s.

In the dissenting opinion for Delaware v. Fensterer, Justice Brennan disagreed with the majority's ruling that a defendant's Sixth Amendment right to confront witnesses was not violated when an expert witness could not recall the specific basis of his scientific conclusions. He argued that this interpretation significantly weakened defendants' rights under the Confrontation Clause and undermined its purpose: ensuring reliable evidence by exposing weaknesses through cross-examination. According to him, allowing experts to testify about their opinions without recalling or revealing underlying facts effectively shields those facts from scrutiny and denies defendants meaningful opportunities for cross-examination. Furthermore, he contended that it is unfair to expect defense attorneys to challenge such testimony without knowing specifics about how conclusions were reached.