I have had a couple questions along these lines, so I present something I have written before. (I have answer a lot of questions here, after all.)
From a reader…
QUAERITUR:
Is it a mortal sin if one accidentally lies in Confession about a venial sin?
You cannot “accidentally” lie. A lie is a deliberate distortion of the truth. It cannot be an accident. People can makes mistakes, that is, they can say things that are false “accidentally”. But people cannot lie “accidentally”. Lying is intentional.
Lying about anything is a sin, more or less grave depending on the matter and the situation.
Lying during confession, however, is grave because of the circumstance.
Lying during confession is also a misuse of a sacred thing, a sacrament, which makes it also the sin of sacrilege. One act (lying), two sins (lying and sacrilege).
How does one confess such a thing?
“Bless me Father, I have sinned. My last confession was yesterday. During my last confession I lied about something. In doing so, I also committed the sin of sacrilege. I deeply regret so mistreating the Sacrament of Penance. I lied about X. Before that my last confession was two weeks ago. The sins I confessed yesterday were these: A two times, B once, and C three times. For these and any sins I cannot now remember, I am sorry. I ask a penance and your absolution. My Jesus, mercy.”
God hates lies. God cannot be fooled. You cannot escape God.
Remember Proverbs 6: 16-19:
Six things there are, which the Lord hateth, and the seventh his soul detesteth: Haughty eyes, a lying tongue, hands that shed innocent blood, A heart that deviseth wicked plots, feet that are swift to run into mischief, A deceitful witness that uttereth lies, and him that soweth discord among brethren.
Of the six things that God hates, two of them concern liars.
Go to confession.
Confess all your mortal sins in both kind (what kind of sin) and number (how many times or how frequently you committed each kind of sin.
Don’t hesitate. Just say it.
Don’t chat. Don’t beat around the bush. Don’t ramble.
Don’t add all sort of extraneous and pointless words or details.
Be blunt.
Omit nothing.
Well,
a lie properly so-called is not accidental, of course.
Still [scrupulosity semi-alert on], is there a point where the negligence necessarily involved in making a mistake and saying a falsehood – which the inquirer probably meant – amounts to culpable disrespect of the Sacrament [off]? [That’s hard to know or pin down. The basic point is that people should do their best in stating the facts and not getting bogged down in elaborate details and rabbit holes.]
(I read that you begin with “I am [age] years old and a [profession] and my last Confession was [date]”, and have made a habit of it: I even got my own age wrong once, by a difference of one.) [No big deal. There are times when I forget how old I am and I have to do some arithmetic.]
I find that almost everyone, at least from 4th grate on up, can understand that God forgives ALL our sins in confession, even the ones we have forgotten, but not the ones we remember but purposely omit. I find that almost everyone understands that for sins of theft, it is necessary to make restitution, even if that is must be done covertly or vicariously.
[Yep. They aren’t hard concepts, but they do have to be taught and refreshed from time to time.]