Late Game Boundary Violation
 

We are to issue a warning if a defender crosses the plane during a throw-in assuming no warning has already been issued.

Team A scores and is now down 1 with 4 seconds left. B is out of timeouts and immediately steps across the boundary without touching the ball or the thrower. I assume by rule we blow the whistle, stop the clock, and issue a warning. If so the defense has gained an advantage by having the clock stopped.

Am I right on my understanding?

No. Let the clock run.

As in "act like I don't see it?"

I believe late in the game, if it doesn't affect the thrower in, you ignore, If there is contact, or in your judgment, (this is why we get big $) the act affected the throw in it is a T plus a warning.

See Case 9.2.10 COMMENT. It addresses the exact play you've outlined. An official is to ignore the throw-in plane violation if its only purpose is to stop the clock, unless the tactic interferes with the thrower's efforts to make a throw-in, and then a technical foul shall be called even though no previous warning has been issued.

Not that we "act as if we don't see it" we just ignore it. The comment under 9.2.10 should explain it for you.

Thanks for the clarification and the case play.

I'm a smarter official.

Here's the case play and note, in case you don't have your case book handy:

9.2.10 SITUATION: A1 is out of bounds for a throw-in. B1 reaches through the boundary plane and knocks the ball out of A1’s hands. Team B has not been warned previously for a throw-in plane infraction. RULING: B1 is charged with a technical foul and it also results in the official having a team warning recorded and reported to the head coach. COMMENT: In situations with the clock running and five or less seconds left in the game, a throw-in plane violation or interfering with the ball following a goal should be ignored if its only purpose is to stop the clock. However, if the tactic in any way interferes with the thrower’s efforts to make a throw-in, a technical foul for delay shall be called even though no previous warning had been issued. In this situation, if the official stopped the clock and issued a team warning, it would allow the team to benefit from the tactic. (4-
47-1; 10-1-5c)

Let's say that the team that just scored and is losing plays aggressive defense, and happens to cross the plane with an arm. The purpose is clearly not to stop the clock, but in an effort to make the inbounds pass difficult.If B has the ball OB and you've started your count, do you have a warning? I certainly do.

If B is trying to inbounds and there's more than 5 seconds left, yes. If either one of those is not true, I'm ignoring it.