In Acosta v. State, 798 So. 2d 809, 810 (Fla. 4th DCA 2001), the district court reversed the conviction for uttering a forged instrument and grand theft because of a statement by a police officer witness that bolstered the credibility of a key state's witness, Riley, who had been involved with Acosta in the crimes. 798 So. 2d at 809. When the officer was asked why a handwriting sample was not taken from Riley, he responded, "Up until that point, everything Riley told me appeared to be truthful." Id. Acosta's motion for mistrial was denied but the court gave a curative instruction to the jury to disregard the comment. Id. The Fourth District held that it is clear error for one witness to offer his personal view on the credibility of a fellow witness, especially where the witness offering the view is a police officer, because of the great weight accorded an officer's testimony. Id. at 810. In reversing, the district court held: Riley was the key witness for the state, and the state's case hinged primarily on her credibility. She was an admitted participant in the crime, but was not charged. She testified that she had not, but that the appellant had forged the check. The state's handwriting expert could only corroborate her testimony to the extent he believed it was "probable" that it was appellant's handwriting. He found significant similarities between the signature on the check and appellant's handwriting, and dissimilarities which could not be accounted for. This expert has various classifications which he uses to explain the strength of his opinion, and this classification was third from the strongest. Because Riley's testimony was crucial and the defense's main emphasis was on her lack of credibility, we cannot agree with the State that the error was harmless or that it was cured by the instruction. We therefore reverse and remand for a new trial. Id.