There's a corollary to the old adage that jurors make up their minds by the end of opening statement and will not budge once they make up their mind. This corollary runs even in the face of scientific evidence backing up the adage.
If you promise something in opening that you will deliver, like say a witness, evidence, or a defense, and you later fail to keep that promise, then you may not only lose. You may also be unconstitutionally ineffective. The good news is that if you are the appellate lawyer, then take a look at trial counsel's opening statement. If it wasn't transcribed ask your client and others who might have watched the trial what trial counsel said in opening.
The Court of Appeals in a recent case, Brown v. State, A07A1245 (Nov. 30, 2007), adopted the proposition from a Federal case (I'll talk about that case below) that "when counsel promises the jury" that they will hear a "different version of the events from what he ultimately presents one may infer that reasonable juries would think the witnesses to which counsel referred in opening statements were unable to deliver the promised testimony."
This is nothing new really. In Turpin v. Christenson, 269 Ga. 226 (1998), the Supreme Court found that trial counsel was ineffective for promising to prove in opening that the victim in a death penalty case was a "drug-dealing homosexual." The Court in McAleese v. Mazurkiewicz, 1 F.3d 159 (3rd Cir. 1993), held that promises made and not delivered can be ineffective assistance of counsel.
Opening really is a tightrope. Tell a compelling story, and you reach the other side faster than you may think. Overpromise and underdeliver, and you may do more than lose. If you are the appellate attorney, be sure to read the opening carefully to see if trial counsel fell.