There could be another sign, at one of the red marks on the extreme right of Wayne's "Windows" picture. Look at pictures 70 - 73 in
this album. At that point, the trail goes up, after passing a window. The descending hiker does not expect to encounter an upward incline, so it is natural to keep going down. So many people have done this, there is a nice use trail now (see picture 72). Last year, this happened on two separate trips, to a member of my own party. The album above documents the first time. A few months later, I was watching for it so it only cost us a minute or so. And before last year, perhaps a half dozen other times--and that's just when I've been there to see it.
Also, it hasn't happened for a long time, but I have more than once corrected people descending toward Guitar Lake, below the section of trail west of the Whitney-Keeler notch.
And what about the chute from Trail Crest, when there is snow? I know there have been quite a few fatalities there, while I can't think of anyone who has died by going down the JMT in error. Seems to me, if I were going to erect only one sign, I would put it here.
But I am personally against such signs. Where do you stop? You don't want people to think "I don't need to pay attention or know about what can go wrong. The signs will keep me on track."
(By the way, the terrain looks steep in Wayne's picture because you are looking down at an angle of about 32 degrees. You can verify that by doing some geometry on a couple of features in the picture.)
