A Sandoval hearing is designed to let the accused make an informed choice whether he should take the stand prior to testifying by providing a pre-trial determination of the permissible scope of cross-examination of the accused (People v Sandoval, 34 NY2d 371). Generally, a trial court's authority to change its Sandoval ruling is limited once defendant has decided to testify in good-faith reliance on the court's pretrial ruling (see, People v Powe, 146 AD2d 718, 719, l).
However, there is an important exception to this general rule which defense counsel needs to be aware of in questioning witnesses. As the Court held in People v Lyon, AD3d, 2010 NY Slip Op 06892 [10/01/10], where "a defendant's testimony conflicts with evidence precluded by a Sandoval ruling, "the defense opens the door' on the issue in question, and the [defendant] is properly subject to impeachment by the prosecution's use of the otherwise precluded evidence" (People v Fardan, 82 NY2d 638, 646; see People v Rodriguez, 85 NY2d 586, 591)." Indeed, in Fardan the Court held that the exception applies not only to the defendant's testimony but also when "a witness for the defense testifies to facts that are in conflict with the precluded evidence."