Sparks flew when I was disconnecting negative car battery cable?

I decided to disconnect the negative cable to the battery because I was going to install a power window switch. While disconnecting the negative cable to the battery, a stream of sparks flew out. I checked inside the car to make sure nothing was on and noticed the radio was on. Was this more than likely the reason... show more I decided to disconnect the negative cable to the battery because I was going to install a power window switch. While disconnecting the negative cable to the battery, a stream of sparks flew out. I checked inside the car to make sure nothing was on and noticed the radio was on. Was this more than likely the reason it sparked? I didn't get shocked or anything just scared the crap out of me.
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