I was (am) under the impression that there are certain sins that can never be mortal -- namely, sins that are not of a grave nature and therefore can't meet that one objective criterion for mortal sin.
It's not hard to imagine a (usually) non-grave sin -- for example, a lie -- as having manifest grave consequences and somehow entailing mortal sin. But perhaps such sins are best understood as a sort of compound sin, entailing both a falsehood intended to deceive (i.e., a lie) and a separate intention of "grave injury to the virtues of justice and charity," i.e., the mortal sin.
It's tricky because the catechism seems to make clear that a lie can be mortally sinful (par. 2484):
"The gravity of a lie is measured against the nature of the truth it deforms, the circumstances, the intentions of the one who lies, and the harm suffered by its victims. If a lie in itself only constitutes a venial sin, it becomes mortal when it does grave injury to the virtues of justice and charity."
I'm not convinced that this rules out an understanding of a lie as strictly being always venial, and the key may be the words "in itself": The lie itself may always only constitute venial sin, and the "it" which clearly *can *constitute mortal sin and which does in fact refer to the lie (but perhaps loosely enough) may be referring to this sort of compound sin.