Urg. Sounds like it must have been their storage. Thankfully most of the time my potatoes go bad they just shrivel up and go sprouty, not gross. But every now and then, catastrophe. Even though I always keep them the same. I mean, I don't have ideal storage, because my kitchen is too warm, but it's not like I store them for months. Maybe it's variation in the way they were treated. I mean, I mostly don't buy organic poatatoes so often they tend to have been treated after harvest with anti-fungals or such.