A more thorough explanation:

Definition: Falconer error is a mistake made by a trial court when it fails to instruct the jury that if they find the defendant guilty of manslaughter, they must acquit the defendant of murder charges. This error is named after the case Falconer v. Lane, which was decided by the Seventh Circuit Court of Appeals in 1990.

Example: Imagine a case where a defendant is charged with both murder and manslaughter. The jury finds the defendant guilty of manslaughter but does not acquit the defendant of murder. This is a Falconer error because the jury was not properly instructed that a guilty verdict on the manslaughter charge requires an acquittal on the murder charge.

Explanation: In the example given, the Falconer error occurred because the jury was not properly instructed on the law. The law requires that if a defendant is found guilty of a lesser offense, such as manslaughter, they cannot also be found guilty of a greater offense, such as murder. This is because the elements of the lesser offense are included in the greater offense. Therefore, if the defendant is guilty of the lesser offense, they must be acquitted of the greater offense. The Falconer error occurs when the jury is not properly instructed on this principle of law.