An invincible error is, in Christian philosophy, a normally sinful action which is not considered sinful because it was committed through blameless ignorance that one's actions were harmful or otherwise prohibited.
In the stated philosophy, a sin occurs when a person knowingly commits an evil act, meaning that they must know both:
If a person is ignorant of one of these two facts, then the type of ignorance becomes important. If the person is intentionally or willfully ignorant, this is known as vincible ignorance, and the act is still considered a sin. If, however, the person is unintentionally ignorant of one of these two key facts, then they are considered invincibly ignorant, and have committed an invincible error.
References
Invincible error Wikipedia(Text) CC BY-SA