I had that happen to me in a double-handed race. We tacked just after the start, my crew hiked, bang, he managed to hold onto the stanchion.
Tacking again and heaving to brought the gunwale close to the water and I dragged him back on board.
The only thing that reduced my guilt at not having checked the guard wires more carefully was that he was the broker / boatbuilder that I bought the boat from, and I had asked him to prepare the boat before launch ...