With one drop target already down, a hit on the lower pop bumper makes switch test show a different drop target switch closing, and the drop target bank resets.

Hitting a pop bumper resets the drop targets: faulty pop bumper coil

Cause: The pop bumper coil (with its diode) was at fault; swapping the coil cured it. A reversed or failed diode on a coil or switch can feed signals into the wrong switch line.

Checks

• Check that the wire on each switch is on the correct side of its diode (banded end).
• Test the switch and coil diodes out of circuit.
• Check whether the false switch closure only happens when that coil fires.

Fix

• Replace the pop bumper coil (and its diode).

Systems

• Williams System 6 / 6A

Games

• Gorgar

Related tips

• A drop target switch registers when the lower pop fires: replace the pop coil
• Pop bumpers do not show in switch test: pull the ring down to test them
• Hitting the left pop bumpers tilts the game: failed PIA on the MPU
• Drop targets sometimes don't score: worn horseshoe contacts
• The switch test cannot check pop and sling trigger switches

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