There’s supposed to be some foam stuffed up in the skull to keep this from happening.

Here's the service bulletin for that. Pick up some poly-foam at Hobby Lobby.

Williams_1991_Terminator_2_Judgment_Day_Service_Bulletin_33.pdf

Other than the foam, I have no idea.

Not sure if the others that said foam in the skull, even watched the video. These are 2 different issues.

I have a similar issue. I'm able to lock two balls in the skull area during multi ball. The second ball is not inside the skull but rather kinda stacked on top and behind the locked ball. If I lock the third ball in another lock, the game loses track of where a ball is and will do a search for the ball and all coils will activate. Is there supposed to be something to keep two balls from entering the skull area? There is the drop target but it falls and doesn't go back up if hit during multi ball. Thank you for insight.