A mistake is possible here, even if it's not likely. Consider the difference between this taser and this one. The first is a different color than any gun I've ever seen any cop carry, the shape is way off, and the safety is on the right. The second is colored like a gun, is closer to the shape of a gun, and the safety is on the left. So depending on the taser this cop carries and the gun she carries, it's not completely unbelievable that this was a mistake (and from the video it does look like drawing and shooting at a moment's notice).

Of course, none of this excuses the pig. She should be in fired and in jail right now even if this was a mistake. It does help to be as accurate as possible when talking to other people about this stuff, though. Details like this are also helpful for determining if the "mistake" story is a lie.