"How can a person mistake a taser for a pistol?"
It's the same reason why a person can miss an easy shot at 3 yards, stress, or rather the absence of continuous training in real-world concepts which can be applied every single day. It is true that most tasers in use, like the X26 police version, are yellow (although they are available in black for the non-police market), it has an ambi-safety which you need to click downward in order to turn it on, it has a green LED readout of its power (battery) which faces the user, it's shaped differently than the Glock pistol (or most other pistols) and when you turn it on a red, or green or both, the laser turns on which is used as the visual target impact of the spread of the leads indicator. Most taser holsters are also different than pistol holsters, like Blackhawk holsters for tasers and Safariland holsters for pistols, they deploy differently with different mechanisms. Beyond all of that, when tasers became prevalent in US LE the BART PD / Oscar Grant mix-up occurred which immediately changed how LEO's were allowed to carry their tasers, they had to be mounted in a cross draw and they had to be opposite of your gun. You now literally need to reach across your centerline in order to draw a taser instead of a pistol, this is also part of the standard teaching course officially taught by Taser.
"I still don't understand how this mistake could have happened."
Let's start with the root of this cause, the absolute beginning of why these things happen from time to time with such rarity we actually remember the departments and names of people involved. Training. Many departments that issue tasers to their officers do so because of liability, much like body cameras and OC spray, and the training which is accompanied with those tools is always lacking. Police budgets for training are always fluid and those who make the decision on what training is more important than another usually disregard taser training, much like body camera and OC training, because the number of bad incidents that stem from their use is so low that it might as well be considered a statistical anomaly, which it is. I can think of four high profile accidental shootings-instead-of-tasering in the last fifteen years, contrast that against how many police-citizen contacts occurred during that time, and further contrast it against how many police use of force against citizens occurred with officers who had tasers issued to them at the time, and this falls right around the statistical probability of being hit by lightning twice while the airplane you are in crashing into another airplane in midflight over the north pole while winning three different lotteries at the same time for over 1 billion dollars each. Probably not that improbable, but it's closer to that than not.
Training is the issue at hand here, training is the reason why these types of things happen or the lack of it. Hypothetically, say you have a department of 100 officers, and each one of them is issued a taser for duty work. That means each one of them needs to qualify on that taser every year through training of watching a PowerPoint presentation explaining the physics of electricity, how tasering works, the technology, the issues, the case law applicable to their particular state and federal circuit court decisions, then doing practical application training. The first time an officer gets this training they may be tased themselves so they know how it feels, but sometimes that doesn't happen due to department policies. Then officers are allowed to shoot their taser at a silver foil target which shows them how the leads/prongs spread and how they should conduct targetting (splitting the waistline whenever possible, and aiming for large muscle groups, as well as no-shoot zones - this is all department-specific of course). So just at the first step of issuing tasers to your 100 person department you have 100x the cost of a taser, you have to shoot at least two cartridges in training, so that's 200x the cost of cartridges and then you need 200x cartridges for the officers to carry on the street. Then every single year you need to have every single officer you issued a taser and cartridges to, requalify, another Powerpoint, another test, another session of shooting the cartridges in a flat range / classroom-type environment. Then you need to get officers and supervisors trained on taser use, laws, paperwork, and maintenance. Don't forget the software needed to get the information downloaded, and some of the tasers have cameras built-in, some have automatic body camera activation technology, etc, lots of options. You also need spare tasers, spare cartridges, etc. That is the best-case scenario for the minimum training requirements for the taser to just start off. So far the cost is huge.
Except some departments don't even do that. They may simulate the taser shooting, they may not even make you go through the motions of drawing your taser since remember, you are also paying 100 officers 8 hours of in-service hours for training. If this was the 1970s the FBI would be investigating this as a pyramid scheme meant to defraud departments because of how prevalent it is and how expensive its upkeep. This is why some departments take shortcuts and don't require officers to ever draw their tasers in training at all, and the only time that officer actually has to draw their taser from their taser holster maybe for maintenance, to remove and replace the cartridges, or in real-life use of force situations.
The majority of departments do not require you to use a taser in force on force training, meaning that if a department even offers force on force with simunitions, they will not provide you with taser cartridges that actually fire or even require you to draw your taser in those scenarios. Those departments that do offer all of the above, multiple times a year (as in quarterly, so maybe 4-6 days a year of this type of training) are extremely rare and should be considered the vast minority of departments in the US. This is definitely not standard in the LE training world. Even in those departments that offer all the above, this means an officer is required to draw their taser from their holster on their duty belt something like 4-10 times at most, per year. This also applies to pistol shooting, rifle shooting, and actually every other type of tool training. Hell, if you think this is bad ask any LEO when the last time they were mandated to train with their expandable baton after initial training and issuing of it?
This is a concept I have repeatedly hammered on every chance I get, and it is one of the reasons why I refuse to burn bad repetitions (reps), regardless of circumstances, because when you are on autopilot (condition black, heart rate above 175 bpm) you do not think about how to draw your gun or taser, you just do what you train to do. The biggest issue in modern LE is the lack of consistent and repeated training on systems, and logical processes, that have been proven to work in the real world. While this absolutely falls on the individual officer and that each officer needs to accept the fact that if they solely rely on their academy, and yearly qualifications or in-service training, they will fail at some point as police department mandated training is never designed around officer safety or real-world application, but liability protection for the department. This is the biggest failure in law enforcement today, caused specifically by weak top brass. The top brass, the chiefs, and the admin desk jockeys in any given department who make policies and develop training programs have no clue as to what the street officer does, this is because they do not work the street and don't want to, they shouldn't be sworn LEOs as the concept of having someone develop training programs and policies for a job they don't, and won't, do is insane in any form. Beyond that, not allowing LEO's to get outside-department training to further their own skillsets is another big issue. Yes, top brass will make officers use their own time and money to seek out training that the department failed to provide them, but hey at least the top brass actually let officers have tasers, right?
"So how do we fix this?"
If you are an LEO who is issued equipment, your department (likely) failed to train you properly on, you should every single day before your shift while in uniform, draw and get into a fighting stance, maybe make a few airstrikes with your tools, do some transitions from tool to tool and backward in the use of force continuum. Don't use your cartridges on your taser every single day, that will be extremely expensive. Remove the cartridges before deploying the taser, but every single time you draw your taser do so with the mindset that you will be using it and intentionally draw it, removing the safety and aiming at something that resembles a person. If your department policies forbid you from turning on your taser every single day or doing a "spark test" then submit a letter to your chief asking them to amend the policy, given the current situation in Brooklyn Park, and to provide a policy for how to train daily with the taser. You should be doing the same thing with your other tools. Every time you put your duty belt on, you should draw your pistol, aiming it at a safe position, unload it and dryfire, make sure you reload and reholster when you are done dryfiring an empty pistol - all done as safely as possible! Same with your ASP, same with your OC, same with your handcuffs, etc. Every single day you should make sure all of your tools are functional, that they are loaded, if applicable, and you should handle all of your tools every single time you put them on your person before going out on duty.
For the responsible citizens who want to try to affect police departments positively, you need to go to your chiefs, your mayor, your township managers, and you need to ask for their training materials, to see what they are doing. You need to see where they are lacking and if they are, you need to ask them to budget appropriately in order for their officers to be trained properly every single year. That may mean the top brass and admin don't get brand new unmarked takehome vehicles with the full star wars lights package, that may mean they don't get those fancy new desks and chairs, it also means that these types of incidents are further moved into the statistical anomaly section. Unless, of course, you want your city to be burned to the ground by the peaceful protestors.
Training is the way, the only way, and training which is developed through logical mindset, through proven concepts is the only way LEOs, and responsibly armed citizens alike, get through the use of any level of force. There is no exception to this rule. Never accept what some police department trains you with, or on, because they are liability-minded, not officer survival-minded. This is especially true in large urban departments, where it is cheaper to bury an officer than payout in a lawsuit. Think about that when you go to training and when you are working to better train yourself.