About one time in five the catapult fires the ball it should be holding for the lock, even when nothing else is happening on the playfield.

Catapult sometimes throws the locked ball early: clean or replace its switch

Cause: An intermittent catapult (cannon) switch briefly reads open, so the game thinks the ball has gone and acts on it; the switch tested fine in switch test.

Checks

• Watch the catapult switch in switch test while tapping the playfield and wiggling the wires; look for any dropout.
• Check the trough switches too, since a trough switch that misreads can cause a similar early eject.
• Look for cracked solder on the switch lugs.

Fix

• Remove the catapult switch, clean it in isopropyl alcohol, blow it dry and refit it. Replace it (17-1067 in the manual) if it still misbehaves; the original part may be hard to find.

Systems

• Williams System 11A

Games

• Big Guns

Related tips

• Knocker fires several times at power-on with a switch number shown
• Pop bumper fires only now and then: clean and gap its skirt switch
• One switch triggers an unrelated switch (phantom closures)
• A switch does not register in the switch test
• Decide whether a phantom switch is in the playfield or the CPU board

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