How have courts treated prejudicial error at trial?

Appellant cites several events at trial which he contends constitute prejudicial error. The first such instance was the admission, over the objection of defense counsel, of testimony by one of the Government informers as to conversations he had with an unindicted co-conspirator prior to the time the evidence indicates appellant was part of the conspiracy. Such testimony is admissible where, as here, the defendant subsequently joins the conspiracy. Hernandez v. United States, 300 F.2d 114 (9th Cir. 1962); Costello v. United States, 352 F.2d 848 (2nd Cir. 1965).