Well, it's happened to me too, Josie, also a long time ago. In 1973/74 John Slee and myself got to the Semi-final of the Billingshurst Pairs and were playing opponents from the mighty Five Oaks team. The match was level at 1-1 and we we slightly ahead in the decider, but their player on the table (and a Five Oaks scorer !). The player put the break down four times and John stepped forward to pitch us into the Final with the bar due, but the scorer waved him back, saying the player couldn't have heard him say one-up. He carried on, got the break back again, and that was that. An independent observer from the Partridge came up to John and me and confirmed that we had been stitched up, but the scorer got really nasty about it - they were after all the Five Oaks and we were no more than a couple of greenhorns. We did not take the matter any further - we would have been wasting our time - and wrote it off to experience.
So what strikes me as wrong all these years later is that once it is established that a mistake has occured, the mistake is allowed to be compounded. I agree with the wording of the AEBBA Rule as far as it goes, ie no penalty (losing a score) to the player if he was not adequately warned. But allowing him/her to keep the value of the extra 'illegal' pot, plus to carry on scoring afterwards is surely allowing too much. A 'what happens next' needs to be embodied into the rule.
As Josie says, an obligation should also be put on the player to keep track of how many times the break is potted. And I too hate scorers who mumble. >:(