The Saturn launch, or really the Saturn period. They could have recovered from the 32X fuckery (which I think is overstated because no one bought the damn thing anyway) had they not butchered the launch, but the Saturn had other serious issues too. The $400 price tag for one, but the architecture made it so hard to program for that most 3rd companies didn't even bother with it. No games means it's going to bomb no matter what.