Matt's comment was where I was going, but after reading some of the other posts here, there can definitely be times when this can cause confusion, and the PRO needs to be ready to fly a flag...

The most common cause of this issue is when a boat catches an anchor line and the mark goes away, usually with the rest of the fleet nearby or on their way. Most of the time, there will still be a "gate" (assuming the mark being dislodged is still floating and visible), and the rest of the boats will go around the other mark, so there isn't too much of an issue.

However, it occurs to me that many times at skippers meetings, statements such as, "If you see a gate, go through it, if not, round it to port."

But, in the true sense of the rules, if the course is identified as a G at the start, and there is no gate mark present, that could get tossed on a technicality in a protest. It would be best to get the mark boat in there with the M flag, assuming time allows, etc.

Mike