Here's what happened with one of mine and a clownfish.
It was a month or so after adding a harem of juvenile clowns to a 90g tank with dozens of different anemones.
I had been squirting thawed frozen food into the water in different locations, to see if I could speed up the clowns getting interested in the btas and other clown-hosting nems.
Happened to squirt some food near a 3" minimaxi.
A Phantom clown rapidly zoomed down at the food, and careened right into the minimaxi.
The anemone immediately closed up around the clown, grabbing it and pulling it down. Couldn't see the fish at all.
I watched this happen, dumbfounded.
Pulled the tank lid right away, stuck my thumb in there and scooped the clown out.
Actually took some effort, I could feel it tearing away from the nem.
It was dazed and wobbly, couldn't swim straight, and still had tentacles stuck to it.
No doubt if I hadn't done something, that clown would have been lunch.
Decided just to watch, wait and see if it recovered.
It started swimming normally after an hour or two.
Tentacles fell off the next day and the welts healed over a few days.
Here's the little guy shortly after I saved him from the jaws of death: