Im fairly certain that the absolution given is applied to ALL sins, even those you forgot to confess by a simple oversight. You are however, to return to confession to confess any sins that you remember. Basically, if you just forgot, then yes the absolution is still valid. If you remember after the fact, then you are under obligation to confess those sins as soon as is reasonable, which means that, yes, you should go back to confession. The only time you would really run into trouble is if you were to withhold confessing a sin that you were consciously aware of…in the case of withholding the confession of a sin you were consciously aware of, then the absolution is completely invalid.
Think of it this way. God, whos love, mercy, and wisdom, is so perfect it is beyond our human comprehension, is fully aware that we are only human, and subject to human imperfections (such as a bad memory). God is not going to punish us for merely being human.