In State v. Walters, 551 A.2d 15 (R.I. 1988), the Supreme Court of Rhode Island held that a police officer could not properly testify about the "trajectory check" he performed on the victim's car that, according to the prosecution, went to show that the defendant had assumed a "combat position" directly in front of the vehicle as he shot at the victim. Id. at 17.
The officer had testified that he reconstructed the path of the bullet by passing a piece of string starting at approximately the point where the bullet would have entered the victim's chest, passing it "through the bullet hole near the inspection sticker on the passenger's side of the windshield," and then taking it out the right fender of the car.
Based on that testimony, the officer inferred that further extension of the rope would "reach the approximate shoulder height of a man five-foot-eleven-inches tall"--i.e., the defendant. Id.
The Court did not hold that the police officer was improperly permitted to testify, but that the trajectory check was unreliable:
Because this trajectory check lacked any scientific basis and because a jury might be inclined to give weight to the apparent opinion testimony of a seasoned and experienced police officer describing his investigative methods, we hold that its admission constituted reversible error. Id. And although the court clarified that the testimony would have been confusing to the jury if it had been permitted under the guise of lay witness testimony (simply placing "unnecessary and irrelevant evidence" before the jury, id. at 18), the Walters Court premised its holding on the unreliable nature of the police officer's testimony, which rested on his subjective opinion about where the bullet hit the victim and his approximation of where further extension of the rope would lead.