Newly discovered evidence
There are formidable legal doctrines, however, that prevent defendants from obtaining a new trial even if they prove that a key witness against them committed perjury. For example, the "[v]eracity of witnesses may not be tested for a second time and by an appellate tribunal." United States v. Rutkin, 208 F.2d 647, 654 (3d Cir. 1953). If the defendant challenged a witness's testimony at the first trial, then this doctrine precludes the defendant from challenging it again in a request for a new trial, even if newly discovered evidence proves the lie.