Does failing to perform the penance somehow make the reception of the Sacrament of Penance invalid? I realize that absolution is always imparted but does the penitent miss out by not doing the penance? How would justice be served if the “restitution” so to speak were not performed?
As Lily clarified rather nicely, if one honestly forgets to carry out their penance, or for some reason becomes incapacitated as regards their penance, then there is no sin and no loss of absolution (just like any validly completed sacrament, once complete it is permanent, nothing UNDOES absolution)
Deliberately skipping your penance MIGHT be a new sin, depending on your knowledge and state of free will (as well as your intent).
The only case that would affect absolution is, as Lily noted if prior to and during confession you determined to abuse the sacrament and intended never to do any penance. Even then, the lack of intent to do penance is only indicative of a lack of repentence, not necessarily declaritive, so the absolution MIGHT still be valid in that case… but why take the risk? One should never go to confession with the intent to skip out on doing penance.
Finally, one clarification: penance is not really “restitution” for sin, since we can’t take back and undo the effects of our sins. Penance is restoration of ourselves and our souls, and brings us into full reconciliation with the church and the community. If we fail to meet our penance requirements, then we will not be restored and reconciled, which means when we die we will spend more time in purgation being perfected.
If you think you’ve forgotten penances to be done, or want to work a bit on your purgation now, you CAN assign self-penances (such as skipping meat on all Fridays of the year, offering liturgy of the hours specially as a penance, etc)