Almost?
Yes, as in it's a rarity to see a pass backwards for player who is offside. Simply because of the way the game flows. But when it happens it is correct.
No, that's a theoretical impossibility..
Edit: well, no it's not I guess, but only when the player was offside and has to run backwards to pick up the backward pass. Not sure where the law stands on that exactly, but I don't think I've ever seen it happen either.. Is that what you are referring to?
That's what I was talking about when I said almost always, in that it's not impossible but very unlikely for the ball to go backwards and yet still be offside.