Entrained gasses may affect our gravity readings. Any liquid at equilibrium with the atmosphere will have entrained gasses which would cause a gravity reading to be (very?) slightly lower than it actually is. We can assume that OG readings using freshly cooled wort that have little or no entrained gasses will be accurate within the tolerance of our instruments, but what about an FG reading?
A finished brew will have considerable CO2 dissolved in it. And let's say that you forget to take an FG, and have to use a carbonated sample from the keg. What then?
This is one of those trivial things that I did not concern myself with in earlier times. It still does not greatly concern me, but my nature causes me to wonder what correction I should make when taking a reading from a finished brew or a carbonated sample.
Charlie