Hitting pop bumpers, slingshots or rebounds advances the bonus multiplier, which should only advance from the mushroom switches. Switch test and diodes look fine.

Bonus multiplier advances on bumper hits: hair-trigger mushroom switch

Cause: One mushroom switch was so close that playfield vibration closed it. Its stack had been assembled with a thin insulator where a thick one belongs.

Checks

• Slip a small piece of paper between each mushroom switch's contacts, play, and see whether the false advances stop.
• Compare the insulator stack order of each mushroom switch with the others.

Fix

• Rebuild the faulty switch stack with the insulators in the correct order and re-gap it.

Games

• Fireball Classic

Related tips

• Pop bumpers won't fire from the skirt, or fire by themselves: bad switch capacitors
• Pop bumper and both outkicker switches dead: bare wire touching the coin door
• Pop bumpers fire nonstop after a solenoid expander swap: board jumper set wrong

Source: Pinball Repair help group!! (Facebook group)