Spontaneous fermentation!!

So I got a few gallons of preservative-free cider from my local orchard that I had planned on making into hard cider. One of these gallons was left in the back of my fridge for prolly a good month. Today when I was cleaning everything, I noticed that the bottle looked a little bigger than usual. My sister took it out, opened it, and poured a glass. Lo and behold, it has naturally carbonated itself! I took a taste and could instantly tell it was fermenting cause of the tangy taste of the natural yeasts.
Has this happened to anyone else? I understand why it happened, figured it would be a fun story to share with everyone.

I did that once and it was not so good! Tasted like vinegar more than cider. It was tossed!