It has actually happened a number of times, I've heard of maybe a half dozen, at least. It's a shame that we can't figure out a way to exclude firing the handgun instead of the tazer, but the handgun is what people will always default to because it is on the right side and is trained with. You must make a conscious effort under stress to reach across and grab the taser instead. It sounds ridiculous, but the reaction would be "oh, no, i gotta taze the guy' and obviously there will be more mental processing being used observing and planning, and deciding what move to make will be given short shrift. A person may think 'I gotta draw and fire (my tazer) and lose conscious thought that he isn't supposed to be grabbing his gun.
Not a great explanation, but it's a similar thing to turning off the lights when you mean to turn up the radio. The mind is focused on one thing. A decision is made to perform another simpler task, one that is easy and really doesn't reach into the consciousness. It's not like accidentally turning on the windshield wipers instead of the turn signal, the consequences are far greater, and the stress and distraction are far greater.
The way you avoid these things is by thinking ahead. If the shooters had gone into that event knowing that there may be a need for a tazer and planned ahead, the reaction of drawing the gun instead could have been prevented. The mindset wouldn't have been just a chaotic grab, the mindset going in would have been 'if I have to I will deploy the tazer that is on my left side.'
I sure don't want to be in that situation. I can't even drive and text, much less decide between a gun and a tazer in a split second when the SHTF.