Not one mention of the immaculate reception? Seems the most likely, and very historic, candidate.
This is something where the I believe the NFL has intenctionally gone out of its way to maintain the mystery where there really should not have been any from shortly after the game ended. Every replay shows the obtructed view of the collision (that led to the deflection that Harris caught) with the camera catching Harris about 15 yards later running unobstructed to the end zone, followed by the end zone view of Harris clearly picking up the ball before it hit the ground. The controversy being if the ball was deflected by Fuqua (Pittsburgh) or Tatum (Oakland).
At the time of the NFL's 75th anniversary (~1994) the firm I was working for at the time was consulting with NBC sports. NBC sports was given a few (then) commemorative video CD's (same size as a CD and DVD, earlier technoliogy than a DVD, could only be viewed on a PC with a CD drive at that time) of the NFL's greatest plays and a VP at NBC gave me one. That CD showed the play from all angles and they had one angle that was clear as day that Tatum hit Fuqua just before the ball got there and it deflected off Tatum's left shoulder, Fuqua did not touch the ball. I highly doubt the officials saw that until possibly long after the game was over and I'm very confident they took the stance that "nobody was in position to call it no TD so they had to allow it", although I do believe that some weight may be given to the Raiders' claim that the refs didn't want to piss off 65,000 Steelers fans at that point. And for the record, not only were the officials unsure at that time if the ball was deflected by Fuqua or Tatum, they likely did not know if Harris caught it before it touched the ground until afterwards.
One last comment; if the are using current rules, allowing technology to determine calls, shouldn't they also be using current rules that would allow an offensive player to catch a ball deflected by another offensive player?