Bent chainstay - trashed bike- uncommon?

Wise men learn from the experience of others, it is said.

I have twice had a chain come off and jam while descending. The first time when I was very young and ultra light but riding a very heavy frame which survived without bending, the second, when I really should have known better, in the middle of a club run group going down the hill from Remenham towards Henley. Wonderful to relate, we all stayed upright. However the damage to the frame was just as described above, but luckily the late and much regretted Dave Russell was still around and he had no difficulty repairing it.
There is no question in my mind that both incidents were caused by allowing the chain to become slack.

I admit to being a fool who has had to learn from his own experience, but I have learned the lesson well, and now I always check the chain tension before taking any fixed bike anywhere near a hill.
Two points to note:

1. A new chain takes a few miles to bed in. Recheck tension frequently (first 10 miles) until it stabilises.
   
2. Allow for tight spots. Most sprockets are not perfectly round, so it is necessary to find the tightest point and set the tension there.