Woah! So apparently there IS a way for some people to pick up unused permits!
From that article:
But on a Sunday this July, the petite college student was back to try again. Only she didn't have a permit. She was hoping that somehow, 14 miles up the trail, just beneath the last stretch before the cables, she might be able to ask the ranger for one on the spot. But first she had to get there.
Later in the article:
Fortunately for Dolores and Galang, there were a few extras that were unclaimed that day. Though rangers advise against anyone making a 14-mile hike with only the hope of securing one of the unclaimed permits, they are available. On average, only 296 of the 400 daily permits are being used. On days when permits are not required, the number of hikers tackling the cables averages 650.
