I tried making yogurt a couple days ago.  I found a couple cups of organic milk that was getting close to the end of its shelf life, and about a quarter cup of active culture yogurt.  I heated the milk until it was about 100 degrees, then added the yogurt and set the whole thing out in the car, where the temps were quite toasty.  After the sun started setting the car cooled off quite a bit, so I heated the oven a bit and put the half-curdled milk/yogurt in for a couple more hours.

About 3 hours later, I had a mass of yogurt floating in whey.  I drained off the whey and was left with a rather runny, tart treat. I stashed it in the fridge for a couple days, then tried it this morning.

First off... despite the age of the yogurt (it was past its freshness date, but still tasted good) I was pleased to find that the chilled product was quite firm.  It has a pleasant tart taste but a grainy texture.

A quick trip on the Web and I found the problem; according to an article by Bob Roberts for Dairyfoods.com (which appears to be a trade website), culturing yogurt at higher temperatures (mine was over 100 degrees) can cause graininess, as can insufficient mixing of the culture in the milk.

Next time, I'm going to toss the yogurt and milk in the blender for a few seconds to assure adequate mixing, and will make all attempt to keep the culturing temps closer to 100, maybe even a tad lower.

I call this a successful failure for the sheer fact that I made yogurt without trying too hard, using less than ideal ingredients (ie not as fresh as they could have been) and it actually tasted like good yogurt.  In the past, when I've attempted making yogurt, I think I heated the milk too much and ended up with something that had a cooked taste.  I didn't get that with this batch and so I'm quite happy despite the graininess.