A score relay or feature relay locks on as if a switch were held closed, though the switch contacts look correctly gapped.

Playfield switch damping blade shorting

Cause: The short third blade that steadies one contact blade is bent and touching the other blade.

Checks

• Look between the blades of each playfield leaf switch for a damping blade touching the wrong blade.

Fix

• Bend the damping blade back so it only supports its own blade; when adjusting a switch with one, move the short contact blade and damping blade together.

Systems

• Electro-mechanical

Related tips

• A switch blade has lost its contact
• Why EM switch contacts get dirty
• File EM switch contacts
• Normally open, normally closed and make/break switches
• Look over every switch before first power-up
• Game stopped working after filing switches
• Bend switch blades at the stack, not along the blade

Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part One, Clay Harrell (pinrepair.com)