Our boats have hit other subs, freighters, fishing boats, buoys, underwater mountains, and run through fishing nets dragging their poor fishermen along with it. Secracy of operations tend to keep these out of the news but a lot of them become public knowledge. Most of them are unavoidable with just a couple exceptions such as the Japanese trawler off of Hawaii. One of my boats drug a French fishing trawler backwards for a couple of miles, 50 tons of trawler don't put much of a drag on a 7000 ton submarine and we didn't really know what happening at first other than our planes were temporarily jammed, our XO picked up on it right away and we shook off the net and came to periscope depth to make sure the fishermen were OK. We did not surface as the boat was intact and the fishermen were standing on the rails with rifles, I still wonder what was going through their minds.
Most of these collisions have mechanisms to help avoid them but the very nature of submarine operations would seem to make an occasional collision inevitable.