If a base coach interferes with a thrown ball, the runner will be ruled out. But if a thrown ball accidentally touches the base coach, the ball is alive and in play. Coaches must respect the fielder's right of way to make a play on a batted or thrown ball.
Base coaches can also be called for interference if the umpire determines that they physically assisted a runner -- by grabbing or holding said runner -- from leaving or returning to first or third base. Moreover, base coaches are not permitted to leave their boxes and act in any manner intended to draw a throw by a fielder.