Bob R's "with dire consequences" phrase above pointed to a post titled "Glissading with Crampons" posted March '05.
Here's the text and image:
It was in April 2000, and two friends and I were descending after a day climb. The top few inches of snow was soft all day so we didn't really need our crampons, but they gave a little added traction while walking. Of course we knew better than to leave them on for glissades, but the snow was so soft and the going so slow that it didn't seem a problem.
The long glissade below Trail Crest was without incident, but the next good one--down to Mirror Lake—was a different story. After about 200 feet I hit an unexpected patch of ice and speeded up instantly. The outer points of my right crampon caught on the ice and my foot everted. My fibula was put into instant compression, resulting in a spiral fracture. I went head-over-heels with my ankle feeling like fire, but had the presence of mind to successfully arrest without crashing into the rocks below.
My friends were kind enough to split the contents of my pack, and I hobbled down to the Portal. I spent the next six weeks in a cast, and it was two months before I was back in the mountains. The bone healed, but there was irreversible soft tissue damage to my foot and ankle. I must now wear a brace, otherwise I cannot walk for more than an hour or so without pain.
In retrospect, I think the wind had blown the soft upper layer of snow away at that spot, leaving the older hard crust exposed. At any rate, it was unexpected.
The picture on the left was taken the next morning, before I went into the ER. The X-ray is a lot clearer in the original, but I think you can see the fracture:
