To add...if this happens with all of the 5 bank targets up, then the first thing to check is your coin switches. Make sure none of them are stuck closed or shorted. Then you need to check the diodes on the 3 bank drops and make sure none of them are shorted (one or more probably is.)

For this to happen it requires 3 switches to be closed and one of them has a shorted diode or no diode at all. As stangbat said the 3 bank series and slam tilt switches are in the same row and each 3 bank drop is in the same row as a coin switch.

What happens is the MPU scans the switch columns one at a time. In column 1 it sees a coin switch closed, the signal travels down the entire row and should be blocked by a diode on the other switches. But it passes through the closed drop target switch and through the shorted diode, and appears on column 4 where it's not supposed to be. Now because the 3 bank series switch is closed the signal travels through that switch and down row 7 which also has the slam tilt. Since the MPU was in the process of scanning column 1 and it now sees a switch closure on row 7 it thinks the machine was slam tilted.

Remember, this description assumes the 5 bank targets are all up (or at least the top one) and no other playfield switches in column 4 of the switch matrix are stuck closed (eject hole, center and lower right side standups.)

Bill