Don't know how I missed this thread earlier
Approximately 3 years ago, I was the first of our group to rappel a short, small pit (30-40 feet deep and 15 feet across) and while getting my rack off the rope a large flake fell from the opposite wall (not the side we were rapelling from) and landed about 3 feet from where I stood. If I'd been a bit faster removing my rack I would have moved directly into the place where the rock fell to allow the next person to come down the pit. This was an "in-cave" drop - not an entrance drop.
In a crawlway once the person 2 people ahead of me had a large rock fall about 2 feet from the ceiling of the crawlway and pin his leg. Fortunately it was not too large for the person behind him to lift up for him to free his foot and more fortunately it did not block the passage so he could come back out.
I've seen breakdown already on the ground move/roll many times. One time in exploring a "virgin" cave one of the members of our group rolled a very large rock onto himself as he climbed through a hole in the floor. It pinned him in the hole. Fortunately for him there were 5 of us above him and we were able to wrap the rock with webbing and pull it off of him so he could wriggle out of the hole.
Several times I've seen cavers dislodge large breakdown chunks on slopes that could have severely injured anyone who had the misfortune to be below them. These were not careless cavers the rocks were just precariously balanced.
I'm also aware of several caves that were dug open more than once when rockfall closed up the dug open entrances.
BTW - I have some friends who were at Valhalla about 3 years ago and they experienced some rockfall while they were at the bottom of the pit. Fortunately no one got hit.