This is a tongue-in-cheek answer, but I make a litre every day, with 4 containers on the go at any one time.
The other day, I prepared my new litre, but got the jars mixed up. I put the new, unmade yogurt into the fridge, and the recently made one back into the yogurt maker.
Later in the day, when I got the yogurt out of the fridge, it was totally liquid, and it took me a while to work out what had happened.
Could this have happened to you?