I was caught out the other night becoming established on an ILS by forgetting to switch the gps annunciator switch from GPS enroute mode to NAV mode (light twin).
Been there, done that. It's definitely a trap for the unwary when flying an older aircraft equipped with NAV radios that are incompatible with a GPS installation. I don't know that there is a technical fix, other than upgrading to NAV radios that are capable of being wired to switch automaticaly when a LOC frequency is selected. Best advice, when you hear "cleared for the approach" make a conscious effort to check that the NAV/GPS sw is selected for the approach you intend to make.