Yes, this is plausible all right. It happened a few years back in Oakland, CA, with a transit police officer. https://en.wikipedia.org/wiki/Shooting_of_Oscar_Grant
Taser training doesn't direct it be worn in a particular way or place. Most cops wear it in the cross-draw configuration so they can use their shooting/dominant hand. It can be worn on the non-dominant side, which takes a lot of practice & training, something like 2000 reps. The taser has a unique thumb-break holster and a switch to arm it using your thumb as you bring it up. And it has to be aimed not unlike a gun. Most people just feel more natural using their dominant hand. I do my non-shooting hand for this exact reason, and after all this time, it still doesn't feel 100% natural despite constant practice.
As for the Brooklyn, MN officer, looking at the video, she even shouted taser, so in her mind it was the device. We'll never know what was in her mind, fear, but it appears she thought it was the taser. Tasers are way lighter than a gun, feel different and most agencies use the bright yellow ones to help avoid this sort of thing.
Its an unforgivable error, no real excuse here.