Section — Wouters, Manual of Moral Theology
Scholion I. On Venial Sin by Accident
289.
I. A sin mortal in itself becomes venial by accident:
From invincible erroneous conscience, as if someone holds the theft of a grave thing to be venial.
From defect of perfect consent (St. Thomas, I-II, q. 88, a. 6; St. Alphonsus, V, 54, 55).
Some add: from an excusing cause, which because of only a slight defect does not excuse from the whole. But the legislator, if he urges the substance of a grave precept, seems always to urge it under grave obligation, so that one who injures a grave precept according to its substance without a truly sufficient cause seems to sin gravely.