BRS actually posted a pic yesterday on FB where they had a file fish nipping at a clam so the clam shut on the fish killing it. Never thought that would happen but it was interesting to see...
I'm surprised it's the first time you've seen/heard about it. I guess I've just been on RC a very long time. I've seen and heard of many unfortunate instances where a curious tang, angel, etc picks on the clam, or even a clownfish hosting a clam, that got closed on, and they both died unfortunately.
Clams are a different experience than most aquarium critters, and like I said, they can be very finnicky, or they can be rock solid. I had a nice crocea that grew huge in a crevice in my rockwork in my 120. I didn't realize that's actually how they live (they're boring clams, so they dig into the rocks). I took some pics when diving of this, I thought it was super cool seeing crocea clams with only their mantle showing at the surface, and they had actually dug themselves into the rock or coral head so that they were entirely encapsulated.
They just have to be treated differently than other things, and they can bite back, but always to their detriment.
Clams and anemones (especially gigantea carpets) need to be handled very differently than most corals/critters. A nice big gigantea will eat all of your fishes except your clownfish given enough time.