If I'm thinking of the same incident, the ball touched the corner flag. The rule re a corner flag being out of play only applies when a ball carrier touches it, not just a ball.

I think you've asked that the wrong way round.

"When I'd a flick pass a knock on? And when is it not?"

When a flick pass (or any pass) hits the ground, it is a knock on if it's forward.
If it finds a player first, it's a pass.

You can't check for a forward pass, but you can check for a knock on.

It looked like a knock-on to me, but the video replay wasn't conclusive. The attacking side usually get the benefit of the doubt, so I don't have any real problem with the final decision.

That nonsense really isnt it, a pass doesnt stop being a pass because the player it was passed to failed to catch it.

What is it in the time between the ball being passed and and it being caught/allowed to bounce? Is it in some weird state of being both passed and not passed and being both a knock on and not a knock on?

It's in the corridor of uncertainty; a known unknown.

Quantum rugby?