I just looked at the knock-on in rugby union law 11:
Knock-on
A knock-on may occur anywhere in the playing area.
It is a knock-on when a player, in tackling or attempting to tackle an opponent, makes contact with the ball and the ball goes forward. Sanction: Scrum (if the ball goes into touch, the non-offending team may opt instead for a quick-throw or lineout).
A player must not intentionally knock the ball forward with hand or arm. Sanction: Penalty.
It is not an intentional knock-on if, in the act of trying to catch the ball, the player knocks on provided that there was a reasonable expectation that the player could gain possession.
The ball is not knocked-on, and play continues, if:
A player knocks the ball forward immediately after an opponent has kicked it (charge down).
A player rips or knocks the ball from an opponent and the ball goes forward from the opponent’s hand or arm.
I'm sure this has changed since I looked at it a few years ago. I seem to recall that for a knock-on to occur, the ball needed to hit another player or the ground. But there's no mention of this.
In fact, according to the law, a k-o only occurs when a player tackles or attempts to tackle an opponent.
Seriously, who writes this rubbish? The definition clearly isn't what everyone understands (even vaguely) to be a k-o. And "forward" is not defined.
The laws for Tag Rugby are better (and I think this is what used to be said in the standard laws) viz:
12.1
A knock-on occurs when a player loses ball possession, or contacts the ball with a hand or arm, and the ball goes forward to touch the ground or another player before this player gains, or regains, possession. Forward means towards the opposing team’s dead ball line.