Yeah rules say that anytime a ball is thrown with a pin missing the ball must be re-rolled, however I believe it is also illegal to not make it known to the bowler who's turn it is so that he can re-roll. If my memory is correct, its considered cheating to not tell someone they have to re-roll a ball due to a missing pin and then say later that that person rolled a ball with a missing pin and let the score count, especially once the scoresheet is signed.
There have been a few times where I've allowed someone to throw a spare ball after a pin had been knocked over by the setter without replacing the pin, however my stipulation was that I would let it stand if they would have gotten the pin on the spare anyway or I would adjust the score accordingly if it was obvious they would not have gotten the pin.
Actually last week there was a similar situation to this in that someone on the pair next to me left a 4-7 but had a slow messenger come over and knock out the 4 just before the setter came down. No one on their pair saw it so they called to have the 4 placed back on the lane and he would throw at the 2 pin leave instead just the 1 pin leave. When I told them that he had a messenger come over and knock the pin out they had a short discussion and all agreed he would keep the 8 pinfall as it was and go from there. No one cheated but they all talked it over and came to an agreement in a respectful manner.
15.5lb MoRich Nsane LevRG (medium and medium-heavy)
15lb Roto-Grip Cell (heavy oil)
16lb Colombia White Dot (spares and extreme dry)
16lb Roto-Grip Neptune (light and dry)
High game: 300
High series: 779