The talk when I was teaching in the sim was there had been an insidious ADI failure, but that's difficult as the flying was essentially VFR. But it was night low level as I understand it (just before my time as aircrew).
This hastened the fitment of another artificial horizon which the 'planners' managed to put on the Isol DC Busbar, thus ensuring with a RCR failure, all the attitude instruments would fail together. IIRC