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I had my first full-batch drain pour this Saturday...

It was a total disaster of a beer so now I'm trying to figure out what went wrong.

It was a simple little stout - OG 1.044. Grain bill was Maris Otter (~85%) , Chocolate Malt (7.5%) , and Roasted Barley (7.5%). It was only about 3 gallons. I pitched a fresh smackpack of Wyeast 1084 (Irish Ale). I didn't do a starter as the gravity was low and the batch was small. Mash was about 75mins at the mid-temp range. I don't have my notes here at the moment but around 67C.

So 2 weeks in I racked it off the yeast cake assuming it would have been well fermented out. I checked the gravity and it was only at 1.024. Not nearly what I would have expected. I racked it back onto the yeast and agitated it a bit to try to get it going. Nothing happened so I added a pack of Danstar Windsor and still no effect. About a week later it had picked up a white film and some funky looking bubbly blooms of grossness. Gravity didn't really change after a few weeks of that, but it was tasting a little off. I considered holding on to it for a while to see what it would turn into but wanted to clear up some space, so I just dumped it.

Anyway, I think it picked up the infection from being racked in and out of the fermenter somewhere, but I have no clue why it was so under-attenuated. The only thing I would guess is that the Marris Otter was fairly old. Would that affect attenuation?

Did you do an iodine test? I'm sure after 75 minutes it should have been converted, but I've found British malts to be a little slower sometimes in the mash tun. I can't think of why the age of the malt should affect attenuation.

How fast did your smack pack bloat?

My money would be on expired/old yeast, but you said it was a fresh pack right? The lag on a solid fermentation might have given something else a chance to take hold.
Also, are you sure your thermometer is somewhat accurate? Maybe you mashed just out of conversion range, or at least at the top end of it?

Yeah I think you might have nailed it with the thermometer. I had been using a new one that was a little on the cheap side. My good original one had gotten wet and wasn't working very well. My last few beers have all finished a bit high, although other than the stout they were all ok.

My good old one seems to have recovered and I used it this weekend so I'll see how that beer finishes. I should try them together and see how far off the new one is from the good one.

I stopped using 1084 because it would never attenuate good for me at all. I also don't have any luck repitching a pack of yeast straight into a beer that's been fermented that far. The only thing I have heard of working is racking onto a fresh yeastcake of a different and more attenuative strain. I think the iodine test might be hard on a stout.