Has this happened to anyone else before?

The controller has worked perfectly ever since I got it, no problems. Then a couple of days ago I left it on my bed, it was off, Xbox was off too, it had been there for a few hours, then when I went to pick it up, the battery pack was boiling hot, as I said, it hadn't been turned on for hours.
I took the battery pack out, the batteries were boiling hot, the battery pack was hot, and the part on the controller that was attached to the battery pack smelt burnt. I left it to cool down for a few hours, then tried putting the battery pack back on, left it for 10 minutes, and it was warm, not boiling hot, but warmer than it should of been. It won't turn on now, so it's ded.

The burning certainly smelt of solder.

So yeah, anyone have any ideas what happened to it? Is it fixable?

Did you ever change the batteries?
If you put the batteries in the wrong way, they'll get super-hot.

I did that with a tv remote one time, burned the crap out of my hand.

Changed the batteries whenever they ran out, so they got changed every so often.

I'm not sure if they were put in the wrong way, I had used the controller previously that day, and I don't remember changing the batteries after using it.