The accomplice statute reads:

“Every person, acting with the mental state required for the commission of an offense who directly commits the offense, who solicits, requests, commands, encourages, or intentionally aids another person to engage in conduct which constitutes an offense shall be criminally liable as a party for such conduct.” § 76-2-202.

The Court says:

Therefore, to convict Jeffs as an accomplice to rape, the State was required to establish that Jeffs, acting with the requisite mental intent, solicited, requested, commanded, encouraged or intentionally aided Steed to have nonconsensual sexual intercourse with Wall. We analyze the jury instructions in light of these statutes to determine whether they accurately stated the State’s burden.

On the consent instruction:

Jeffs argues that the instruction erroneously focused the jury on Jeffs’ actions and position of special trust, rather than on Steed’s, for the purpose of determining whether Wall consented to sexual intercourse. We agree that the consent instruction was erroneous.

The instruction that was given:

An act of sexual intercourse is without consent of a person under any, all, or a combination of the following circumstances: 1. The person expresses lack of consent through words or conduct; or 2. The person was 14 years of age or older, but younger than 18 years of age, and the actor was more than three years older than the person and enticed the person to submit or participate; or 3. The person was younger than 18 years of age and at the time of the offense the actor occupied a position of special trust in relation to the person.

As clarification, the court also gave this instruction:

In order to find the victim’s lack of consent because the victim is younger than 18years of age and at the time of the offense that the Defendant occupied a position of “special trust” in relation to Elis[s]a Wall you must be convinced that the State has proven beyond a reasonable doubt that the offense was committed by a person who occupied a position of special trust in relation to the victim.

. . . In order to find that Elissa Wall was enticed the State must prove beyond a reasonable doubt that the Defendant lured or induced a person to submit to or to participate in an act of sexual intercourse. (Emphasis added.)

The Court finds:

Jeffs argues that it was impermissible for the court to focus the jury on his own position of special trust and on his own enticing actions in determining whether the intercourse between Steed and Wall was consensual. Rather, Jeffs contends that the jury should have been asked to consider whether Steed was in a position of special trust and whether Steed enticed Wall. We agree with Jeffs.

...In summary, we hold that the term “actor” as used in subsections (10) and (11) of Utah Code section 76-5-406 refers to the person who engaged in the act of sexual intercourse. As a result, those subsections could not be applied to Jeffs. Only Steed’s position of special trust or Steed’s efforts of enticement were relevant in determining whether Wall consented to sexual intercourse. Because the consent instructions told the jury that defendant Jeffs’ position of special trust and defendant Jeffs’ enticement of Wall could give rise to a lack of consent, they were erroneous.

As to why it matters enough to get a new trial:

Because no special verdict form was employed at trial, the jury was not required to indicate the basis for its finding that the intercourse between Steed and Wall was nonconsensual. We therefore cannot determine with certainty whether Jeffs was convicted on the basis of the one valid theory, the two erroneous theories, or on some combination of the three. And because there was no real dispute at trial that Jeffs was in a position of special trust with respect to Wall, a theory we have held to be erroneous, it is highly likely that Jeffs was convicted on the basis of an erroneous theory. Such a likelihood requires reversal of his convictions and a remand for a new trial.

The court also held the accomplice error was faulty with regard to the mens rea element.

Jeffs asserts that he could not be convicted as an accomplice to the rape of Wall unless the State proved that he intended that Steed rape Wall. At trial, Jeffs unsuccessfully requested a jury instruction stating that, in order to reach a conviction, the jury must find that Jeffs intended that the result of his conduct would be that Allen Steed rape Elissa Wall.” We agree with Jeffs that he was entitled to the requested instruction.

As to why:

...Jeffs argues that one cannot be found guilty as an accomplice unless he has the required mental state for the underlying crime to be committed. In this case, Jeffs argues that the jury was not adequately instructed of this requirement.

...The accomplice liability statute ... mandates that the defendant, in this case one who acts as an accomplice to rape, undertake his actions intentionally, knowingly, or recklessly. But intentionally, knowingly, or recklessly in regard to what? The obvious answer is that he must act intentionally, knowingly, or recklessly as to the results of his conduct. And in order for criminal liability to attach, the results of his conduct must be a criminal offense.

In other words,

“A defendant can be criminally responsible for an act committed by another, but the degree of his responsibility is determined by his own mental state in the acts that subject him to such responsibility, not by the mental state of the actor.”...“[I]t is not necessary for the accomplice to have the same intent that the principal actor possessed as long as the accomplice intended that an offense be committed.”

...“To show that a defendant is guilty under accomplice liability, the State must show that an individual acted with both the intent that the underlying offense be committed and the intent to aid the principal actor in the offense.”

The point:

Without Jeffs’ proposed instruction as to intent, the jury could have convicted Jeffs if it found that Jeffs “intentionally” did some act, and such intentional act unintentionally “aided” Steed in having nonconsensual sexual intercourse with Wall.

This is reminding me of Robert Blagojevich's argument. He asked the Court to instruct the jury:

If a defendant performed acts that advanced a criminal activity but had no knowledge that a crime was committed or was about to be committed, those acts alone are not sufficient to establish the defendant’s guilt.

The Government, in the charge of aiding and abetting a conspiracy, wanted:

A defendant is guilty of aiding and abetting a charged conspiracy if he tries to help the conspiracy succeed by committing an act in furtherance of the conspiracy and had knowledge of the conspiracy’s purpose at the time he commits the act. One can aid and abet a conspiracy without necessarily participating in the original agreement. One who aids and abets a conspiracy is guilty of conspiracy.

In any event, Jeffs gets a new trial.