Mark,
The approach to # 13 at GCGC is even more deceptive because the 13th green slopes away from you and the 15th green, directly behind the 13th green slopes toward you.
I've had dozens of guests ask for the yardage, select a club, and scream when their ball comes down 30-50 yards short of where they perceived the 13th green to be. When I remind them that the near flag is on the 13th green, they feel embarrassed because they totally lost their focus on the intended target due to the strong visual signal sent by # 15th green and weak signal sent by the 13th green.
It's a wonderful feature.
Perhaps someone can post a picture of it.
The hidden nature of # 13 green, due to its fall away slope, is a really neat feature