At ORA it was because two pairs - one ocellaris and a clarkii - laid eggs at the same time and sperm from the male of one pair got over the water to the eggs of the other pair and fertilized some of them. So that wasn't a mixed pair.
About the wild - things like at ORA may happen in the reef as well. But I'm kind of skeptical that a clarkii would pair up with an ocellaris or percula.
They may however pair up with similar species like for example chrysopterus if no other of their species is available.
On a personal note: I would never intentionally crossbreed different species - I think it is just wrong.
Today I can't go to the fish store and get a pure percula, ocellaris or clarkii. Most of what they have are designer clowns. And with those tankbred pure ocellaris or percula I always wonder if they are truly pure or some of the designer clown castoffs that just look normal.
That's why I prefer wild pairs for breeding - with the only exception of A. latezonatus where the wilds are just way too hard to keep alive.