Various experiences, all in European 470 regattas.  Only been yellow flagged once, for rocking upwind in about 5knots. Thought it was very harsh. My helm was leaning his torso in and out about 6 inches or so as we went over each wave, purely to try and keep the boat constantly upright.   We sought advice from other teams and they said the best solution is for the helm to sit in the bottom of the boat in these conditions, then there's no way this can happen. Or sit very, very still.

Another time a Polish dude got yellow flagged right in front of us on the run and we had no idea what for. Most times though, it's obvious, the offender pretty much knows they've done wrong and the judges are very good.