The admission of evidence that a co-defendant or conspirator has pleaded guilty, or been found guilty, is inadmissible in the trial of another co-defendant or conspirator, and if such evidence is introduced by the prosecution, then it is error.
If no objection is interposed, then we review such error under the plain error doctrine. If a proper objection is made, then we review it under the harmless error rule. Capshaw v. State, 11 P.3d 905, 913 (Wyo. 2000); Kwallek v. State, 596 P.2d 1372, 1375-76, (Wyo. 1979).
The standard of review for an issue such as this was more definitively enunciated by this Court, though the gist of it had always been clear.