As soon as you realize that your opponent has gotten the requisite games to win the only ethical honorable thing to do is Shake his hand and pack up your cues.
It's ridiculous that people will defend someone knowingly accepting a win they don't deserve.
This Attitude contributes to the negative image of our sport and to the degradation of society as a whole. Everyone should strive to be honorable in life and encourage it in others, not screw your neighbor for 25 dollars.
Jaden
No way!
If the issue was addressed earlier in the match I would most likely give the guy the rack. However, if he's sitting there on the hill and it is then discovered that HE should have marked another rack then there's no chance I'm giving him that rack. You have to earn the victory -- not back into it.
For many players there is more pressure associated with being on the hill and who's to say that he would have still managed to get out had he known what the score should have been.
To me, this is no different than my opponent complaining about a questionable hit earlier in the match that could have changed the score had it been called properly.
Now as far as sportmanship goes, I would have addressed the issue right after the break of the missed rack. I've done this several times. I imagine I've even screwed myself a time or two by asking my opponent if I had already moved my coin or bead.
Maybe it's just me -- but you gotta win that last rack.