Why does it matter if prosecutors ‘tweaked’ testimony about ‘Scary Movie’? What does that have to do with Carlos Coy’s case? What does it have to do with getting him a new trial?
The state should only be presenting the truth; if their case rests on the word of a notorious liar, or someone who can’t remember what happened very well, it's weak. In cases where there is no physical evidence, a jury is basically making a determination of who they trust the most to tell them what happened.
They have to decide if a person is reliable, in order to decide how much weight to give their testimony. If they are not allowed to see that a person changed their testimony, or misremembered facts, they’re going to assume that person’s testimony carries more weight than it should. They're also supposed to take into account if a witness has motivation to lie.
If a jailhouse snitch testifies against his cell mate in return for a lighter sentence, that has to be disclosed in court because it’s a reason to lie. A case that has come up a lot as an example of this is Napue vs. Illinois.
A quick, paraphrase summary:
Napue is on trial. The prosecutor tells Hamer, an accomplice, “If you testify against Napue, we’ll hook you up and reduce your current prison sentence.” Hamer recognizes a good deal when he sees one, and testifies against Napue. During this testimony, the prosecutor askes Hamer point blank:
“Did I promise you any kind of reward for your testimony?”
Hamer says “No.”
Prosecutor says “Are you totally sure about that?”
Hamer says “There’s nothing in it for me, I’m testifying because I want to.”
Napue gets sentenced to 199 years and both men return to prison.
Years later, the prosecutor files some kind of legal document saying “Hey, I promised Hamer that the state would help him if he testified against Napue, so let’s do that now.”
Napue files a court document saying “WHAT THE FUCK.”
The case is reviewed. An Illinois trial court says “Ha ha, FUCK YOU! This lie, this perjury, had no effect on the outcome of the trial. You stay in prison.”
The Illinois Supreme Court says “Uh…That might be wrong.”
Hamer’s testimony had nothing to with the crime, but if the truth had come out before the verdict the jury would have seen that first of all, he was a liar, and second, he was testifying in order to save his own ass. But they didn’t see that; they saw what the prosecutor wanted them to see, which was a stand-up guy dropping a dime on his buddy because it was right, not because he got anything out of it. As far as I can tell the problem was that the prosecutor allowed this guy to lie. The Supreme Court said it was his responsibility to correct the lie, so the jury could see the truth, and he didn't do that.
What did the jury in Carlos Coy's case see?
The court, the prosecutors, the judge, they got to see the testimony change from "saw the whole thing" to "only saw a little bit" to "Yeah, sorry, saw the whole thing". They saw it go from "at the house" to "in the theater".
But the jury didn’t get to see the change-up. It looks like all they saw was Mary and Jane testifying to one story that matched up. They couldn't make a determination of reliability based on what happened, but only on what they were allowed to see.
The principle that a State may not knowingly use false evidence, including false testimony, to obtain a tainted conviction, implicit in any concept of ordered liberty, does not cease to apply merely because the false testimony goes only to the credibility of the witness. The jury's estimate of the truthfulness and reliability of a given witness may well be determinative of guilt or innocence, and it is upon such subtle factors as the possible interest of the witness in testifying falsely that a defendant's life or liberty may depend.