In a recent match race regatta, an interesting (I think) question came up. 
In the NOR, there was a provision that states:

That language is not unusual.
Often there are two different windward marks in the water of different colors. A change mark (Green or "G") that may have been put in place for the next pair to start the upwind while the original mark (Red or "R") is still in use by the pair currently on the leg. Appropriate signals are made at the start or leeward mark to tell the boats which mark they are rounding, of course. But teams sometimes miss the change.

When our pair is supposed to round mark G, mark R is not a mark of the course for them nor is it an obstruction.
But if both boats round the wrong mark, R, the race is to be scored as if they had sailed the course--that is, "as if" it is a mark of the course. 
The question is, as an umpire, do we treat the mark as if it is a mark of the course on the water for their rounding? Most importantly, does rule 18 apply?
(For those who don't do much match racing, marks are rounded to S  and rule 18 does apply to boats on opposite tacks on a beat. Also, a starhoard inside boat does have to tack if her proper course requires it). 

I was working with an International Umpire and we discussed it quite a bit on the tow in. It was then discussed in debrief with two National Umpires.
One NU said she would umpire it as though they are rounding a mark of the course. As soon as it became obvious that they were both treating it as a mark and knowing that it would be scored as such, she would apply rule 18.

The IU (and I) thought that the boats cannot make it a mark of the course by their mistaken action. The scoring decision is an after-the-fact designation. As such it does not make R a mark of the course either. He would call it without applying rule 18.

Things could be really complicated if the incorrect mark is left-most and one (or both) of the boats then goes on to round the correct mark (perhaps realizing their mistake). Obviously I would apply 18 to the correct mark. But now its application to the incorrect rounding could set up some really messy redress issues. I don't even really want to start cataloging the various scenarios. In most of them, I would rather explain to the competitors later that they were not entitled to mark room at that "mark" than explain why I gave an inside boat an advantage to which they were not entitled. 

Since two highly experienced umpires couldn't agree, I thought it would be an interesting one to put out here. Again, for those who don't do match racing, make sure you look at appendix C for the rules that would apply.