If a person tells a lie in confession does that make the confession invalid?The sins that he confessed with a contrite heart will be forgiven. However, if he lied and said he was sorry for a particular sin when he really wasn't, then that sin will not be forgiven.
What happens if a person has been going to confession for years but didn't completely understand the nature of the penance afterwards part of it, and didn't complete all of the penances related to his/her confessions? Are those confessions invalid? Does the person need to re-confess? What if the person cannot remember many of those sins, even in general?Your absolution does not depend on whether or not you perform the penance that came with it. This is because the absolution involves your relationship with the Lord, whereas your penance involves your relationship with man and the Body as a whole. You can't even receive a sacramental penance, or perform one with merit, until you are absolved of your sin. So, he does not have to re-confess. What he will need to do though is to confess that, out of ignorance, he did not perform his penances in the past and to ask for the Lord's pardon. This is a sin, even though he was not aware of it at the time. Now that he is aware, he will need to confess it.
UPDATE (8/22/11): Brother Thomas left a comment on this post shortly after it was posted in which he made various corrections to what I have written above. At first, my intention was to engage his comment, but I never got around to it. Now, so many years later, I see that he is very much right in what he had to say, and I find no reason to disagree with him. His comment appears below:
Your answer is somewhat lacking. Certainly you are right that the failure to do the penance after receiving absolution neither negates nor invalidates the sacrament. The sins are forgiven and the person does not have to confess them again.
But it is not exact to say that the nature of penance doesn’t concern one's relationship with God, but only man and the body. All sins affect our relationship with God and, accordingly, with our neighbor. The Catechism says that “sin also inures and weakens the sinner himself, as well has his relationships with God and neighbor. Absolution takes away sin, but it does not remedy all the disorders sin has caused” (CCC 1459).
This is where penance comes in—it’s a matter of making satisfaction to remedy the disorder in our relationship with God AND neighbor. So penance does concern our relationship with God and not just our neighbor or the Body of Christ.
More importantly, it's not exactly correct to suggest that a person who does not understand the nature of penance should confess as a SIN his not doing the penance before.
If a person is in true ignorance, then there is no sin. Absolute ignorance eliminates all culpability in this type of sin (cf. CCC 1735 and 1860). Now, if the person is purposefully keeping himself in ignorance, that’s a sin. If a person knows he should do the penance but doesn’t, he should confess that on the next confession but it still does not invalidate the initial confession for which he did not perform the penance.
But generally the Church has never suggested that a person should confess as a sin something he did in the past even though he was unaware at the time it was a sin, even if at a later date he becomes aware that he was sinning. The only time the such a confession would be called for is with principles of the moral/natural law written into the heart of man (cf. CCC 1860 again).
For example, a person living a life of fornication may, at the time, implicate culture, social pressure, addiction, and even some ignorance ("I was never taught better"), but once they have a conversion come to realize that they were lying to themselves and they will rightly confess the sin of fornication after the fact since they have come to a greater awareness.
But such is not the case with particular ecclesial norms (such as performing penance after absolution). This norm is not written into the human heart, and, therefore, complete ignorance of it removes the possibility of a sin and need not be confessed later.