What is the probability that a rope will push a carabiner against the wall and it will open? Very little. However, it happened to me when Nicky was lowering me down in the climbing hall last Tuesday. Luckily, it was not the first from the top, so I didn't get hurt.

Markus has a book about accidents in mountains. I didn't have time to read it, but I guess it gives a good impression of what can happen because of some small errors.