Now and then the game never ends: the ball count keeps going up past the set number of balls.

Game keeps adding balls (ball 4, 5, 6...): factory reset and new battery

Cause: Corrupted settings in memory, with a dead backup battery on the board.

Checks

• Empty the trough and check every trough switch in switch test.
• Check the memory backup battery voltage.

Fix

• Do a factory reset.
• Replace the dead battery so settings are kept.

Systems

• Data East CPU Version 1/2

Games

• Secret Service

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