That's a pretty long time to store an egg. It probably had some imperfection in it that let bacteria in to grow. You're lucky it didn't explode and spread the yuck over all the other eggs. :)
You get a bad egg now and then. This is one of the reasons we should crack each egg into a small bowl before adding it to a batch for cooking.
How old was the egg and how had it been stored?
I always suspected that the bad eggs I had from my in-town flock had been ones that started to be...