In State v. Russell, Montgomery App. No. 21458, 2008 Ohio 774, this court stated that because an attorney may not express a personal belief or opinion as to the credibility of a witness, citing State v. Smith (1984), 14 Ohio St.3d 13, 14 Ohio B. 317, 470 N.E.2d 883, it was improper for the State to elicit from its own witness that she had agreed to take a polygraph test and that her plea agreement with the State was contingent upon her passing that test. However, we did not find that the admission of that improper vouching evidence amounted to plain error, because other witnesses testified to the defendant's involvement in the crime, and we could not discount the possibility that the jury might have found this witness credible even absent the testimony about her willingness to take a polygraph examination. Id. at P122.