All crank it ups are single ball modes. If you are getting multiballs for any of them something is wrong. Their is a ball save for CIU so it's possible something is making the game think you've drained a ball and it's kicking out another. The through should be able to keep better track of that though so my guess is something is wrong there.

When I unboxed my MET, from day 1 if I hit the mystery hole while ball save was active I would get another ball.

Switch on the Mystery Hole was wired backwards from the factory and the Mystery Hole was reading itself and an outlane. What was weird was that it involved the inline drops somehow and we went down that rabbit hole before figuring it out. The diode was correct so I missed that the wires were swapped for a while.

If it doesn't always happen when you start CIU then keep track of what else is closed (drop, snake, etc) when you have this occur. Trough also a good suspect.

At least this was the case for me.

I am currently borrowing Metallica from a friend and the CIU multiball issues happened to me too, maybe 2 or 3 times. I was pretty sure CIU was always supposed to be single ball play, so I’ve been wondering how this happened too.

I have seen this maybe 2 or 3 times out of a couple hundred plays. I have magnetized ball issues that occasionally get stuck in the trough and cause issues during other multiballs; either too few or sometimes too many launched out. I had dismissed my CIU MB phenomenon to be related to this happening at the start of the mode with the ball save active.

Never seen that on mine. CIU modes are definitely single ball play. Trough switch issues are often to blame for issues like this but would think you would see more issues during gameplay. Do you get the right number of balls on all the normal multiballs? For cross and snake mb you should start with two balls and be able to add a third. For sparky mb, you start with three and can add a fourth. Coffin is a four ball mb. Does the game ever end a ball with it still in play?