Hi ernorman
I'll be careful if I were you and notify them of the error. I have read on this forum (I can't remember exactly who posted it) about a couple of people being in trouble for not reporting the error. I believe one was in trouble at his citizenship interview. The IO denied him saying he applyed earlier than he should have (the poster used the date on the GC to calculate the years and the IO was using the correct date.) And another one, I believe said they took his GC away for not reporting the error.
I am not sure which threads I read (it was a while ago), but do your reseacrh and try to find those posts. I am not trying to scare you (or I am not even 100% sure what the law is regarding this), but I know I read somebody's post about this, and i am just trying to tell you to research it further.
Good luck.