Yes. An instrument 'internal failure', as happened with a doomed DC-9 in Switzerland, can show you on course even when the Loc "CDI" has failed. There might be no 'NAV' flag, or such.
At DTW Metro (DTW), we had a false glideslope appear a few miles before we intercepted the actual glideslope, but this appeared on the ADI but not on the HSI's secondary glideslope. Fortunately, the ceiling and visibility were not near minimums.
But next time?
