From the description:
NOTE: Some players report that certain games which use magnets that rapidly move the ball back and forth over the magnet (Dialed In, some modern Stern games) can cause chrome pinballs including the Ninja ball to become magnetized over time, possibly leading to balls sticking together. This does not happen suddenly but over 100+ games. Many buyers have had no issues with the chrome balls in most magnetic games, but your mileage may vary.