Throw-in confusion

[rfp]

Many lines on the court, one of which is parallel to the end line. After a made basket by Team A, Team B goes to inbound but instead of going behind the end line, goes behind another line he obviously believes to be the end line and passes the ball in to a teammate. No pressure by Team A. It's not an immediate violation, but it's not a valid throw-in. At what point does it become a violation? Or, upon recognizing the mistake by Team B, do you just blow the whistle and let them re-administer the throw-in behind the correct end line? Would you treat this differently if there were pressure by Team A?