I've been flying hawks for a long time and last weekend in heavy wind my female harris got stuck in a tree about 20ft up, with one jess tangled round a twig.
I was using thin kangaroo leather field jesses and she had new flexible ones fitted a week before. The end of one seemed to have whipped round this twig and locked on itself.
I tried to climb the tree but couldn't get close enough to her and was just at the point of phoning the farmer to see if he could fetch a ladder when the twig broke. Lucky it was dead wood.
So a lesson learned - a little bit of stiffness in your jesses(and elsewhere so the wife says) can be a good thing.
Andy