In the case of a murderer trying to kill your children or something, you can use mental reservation that sounds a lot like lying.
If he says "Where are the kids?" you can say "Not here." If you're in the bathroom and the kids are in the living room downstairs, it's the truth, isn't it? According to the Catholic Encyclopedia article on mental reservation, you can even say "They're not at home," with the implication that "they're not at home for you to see" or something like that.
Catholic Encyclopedia on Mental Reservation --
"According to the common Catholic teaching it is never allowable to tell a lie, not even to save human life. A lie is something intrinsically evil, and as evil may not be done that good may come of it, we are never allowed to tell a lie. However, we are also under an obligation to keep secrets faithfully, and sometimes the easiest way of fulfilling that duty is to say what is false, or to tell a lie. Writers of all creeds and of none, both ancient and modern, have frankly accepted this position. They admit the doctrine of the lie of necessity, and maintain that when there is a conflict between justice and veracity it is justice that should prevail. The common Catholic teaching has formulated the theory of mental reservation as a means by which the claims of both justice and veracity can be satisfied.
Still curious about my original question. Telesphorus brought it around to a veiled reference to his situation at SSPX
Zenith, you say it's a sin to pretend not to know that you ate your sister's blueberry pie, but keep in mind eating a family member's pie is not a sin or theft. We're not talking about covering up a crime here. Let's say there's a good motive to conceal the eating of the pie, like that your sister has a bad temper and will probably commit a sin of anger, trying to gouge your eyes out while blaspheming heinously, if she knows you ate it.
Is it ever a lie to simply not reveal information by pretending not to know something? If you ask "Someone ate your pie?" and you know full well you ate it, is that deception? Or could it be seen as just neutral? Is it only a lie if you say flat-out "I don't know who ate your pie"?
Maybe a better response to your sister asking after the whereabouts of the pie would be, "Someone ate your pie" spoken in a flat monotone, but which she will ASSUME is a question. Or you could say "So you're saying someone ate your pie?" If you are just silent, though, she'll know you ate it.