Well, to add another twist, one of the 2 beers was fine until 3 months after bottling. I remember reading a story in Tony Magee's book about a similar problem they had at Lagunitas once. As I recall, it turned out that some less than ideal lab practices had allowed some extraneous organisms into their yeast culture, and that under just exactly the wrong conditions, with just the right population density this organism, would, after a certain amount of time, begin to further ferment the remaining sugars.
Looking through my records, the yeast for both batches was purchased at the same time, so it probably was all from the same production lot, so it's not impossible that defective or compromised yeast was the culprit. Not to suggest that is the most likely answer, but it's worth considering. And yes I did hydrometer test the Porter and the final gravity was down by a single point, that is within what I'd consider the margin of error.