Throw in situation:

[Nevadaref]

Originally posted by BktBallRef

Yep. Since A1 is legally OOB, in this case, he isn't violating and niether is B1. But as dude pointed out, A2 would cause a violation.

If we wish to be precise, this isn't exactly correct. In both cases it is not a violation on A1 to be touching the ball, since he has not yet released the ball on a throw-in pass, and therefore, the throw-in has not yet ended.
Now, obviously B1 has done nothing wrong if he grabs the ball after A1 holds it across the boundary plane.
But if A2 grabs the ball, it depends on whether or not A1 releases it. If A2 merely touches the ball and then lets go while A1 continuously holds onto the ball, no throw-in provision has been broken and there should not be a call. The throw-in simply continues.
However, if A2 takes the ball out of A1's hands, then A1 (not A2) has violated. Provision 9-2-2 has been broken, since A1 has failed to pass the ball directly into the court. Handing the ball to an inbounds teammate is not the same as passing it to him.