Lets take a look at 5 of Goku's mistakes that might have been for the best.
So start off Dragonball Z with a bang, we are introduced to Goku’s Saiyan brother Raditz. He was sent to earth to verify if Goku had fulfilled his purpose as a killing machine and annihilated all of the beings on earth. And in the case that Goku had failed, he would have finished the job himself. Not long after arriving on earth, he kidnaps Goku’s son Gohan and intends on making good use of the Saiyan offspring. Goku joining forces with his sworn enemy Piccolo and pursues Raditz.
When they all engage in fisticuffs, Goku and Piccolo quickly learn that Raditz by far outclasses their combined efforts on the battlefield. Goku, remembering a Saiyan weakness successfully grabs Raditz’s tail, paralyzing him and leaving him open for attack.
Here is where Goku’s kind heart comes into play. Raditz pleads with Goku to release him, and in exchange would agree to change his ways and flee, which of course was a deceptive ploy. Goku foolishly releases his grip and allows Raditz to continue to give dish out the beatings.
Why is this a great Mistake?
Letting Raditz go resulted eventually ended in Gokus death and in the eventual death of his friends via Nappa and Vegeta. Had they had killed Raditz the first time, this would have meant that Goku and Piccolo may not have been strong enough to defeat Nappa and Raditz. Thus ending the world.
Dying allowed Goku to train with King Kai, return home, defeat Nappa and do enough damage to Vegeta.
His death also resulted in Gohan being trained by Piccolo, which then allowed Gohan to be able to defend himself and prove to be of some use down the road.
And last but not least, had they defeated Raditz at that time, it could have resulted in Piccolo returning to his original plan of defeating Goku and remaining a villain.
Eventually after the defeat of Raditz, Vegeta and Nappa appeared. With Goku having been trained with King Kai, he returned to try his hand at taking on the new Saiyan invaders. Making quick work of Nappa, he later found himself falling short against Vegeta. With some well executed teamwork they were eventually about to put Vegeta down. Krillin being the only one of use still able to move, decided to take the opportunity and use Yajirobe's sword to finish Vegeta off once and for all. But moments before what would be a final blow, Goku, with his kind heart pleaded with Krillin to let Vegeta live. Hesitant Krillin respected Goku's wishes. Vegeta barely able to move, found his way into his space pod, from which he came, and made his way off planet earth with the promise of revenge.
Why is this a great Mistake?
Obviously most of us DBZ fans could agree that this was a fantastic decision. We generally love Vegeta, and he has become one of the most memorable anti-heroes known. But lets take a moment to think about what life would have been like if Krillin were to successfully put an end to Vegeta.
After Krillin pierces Vegeta's body you see him take his last bloody breath as his eyes glaze over. Goku is at home, in bed healing. With their friends still dead, Krillin, Gohan and Bulma still take it upon themselves to go to Namek in search of the Dragonballs.
Upon arriving on Namek, they might successfully come across some of the Dragonballs, but in the end come across Freeza's minions, inevitably meeting their demise. In this scenario, Vegeta was not there to team up with them. Vegeta was not there to buy them some time until Gokus arrival. Goku would have arrived on Namek to a missing Gohan and Krillin and possibly Bulma.
You might think that him finding out his son and best friend were dead would be enough to send him into a Super Saiyan, but I would argue that it was the moment of how Krillin died that was the catalyst for Super Saiyan. If at any point, Goku came across the Ginyu Force, Captain Ginyu would have most likely been successful in acquiring his body.
In the end, Vegeta was a key factor in the survival of Namek.
After the destruction of Namek, there was a period of time where Goku was residing on the planet Yardrat. While there the natives taught him the technique known as Instant Transmission.
Around the same time King Cold, the father of Freeza, found his damaged son and rebuilt him into a cyborg version of his past self (Mecha Freeza). Hell bent on revenge against Goku, both King Cold and Freeza made set their sights on Earth. Upon arrival, they were met by Future Trunks, and after a short battle both met their demise. Later when Goku arrived on Earth, he states that he was going to Instant Transmission to Earth to defend the Earth from Freeza, but decided against it when he felt what was going on during the Trunks engagement.
Why is this a great Mistake?
Lets think about what might have happened if Goku were to Instant Transmission to Earth to take Freeza on one more time.
Gokus anger towards Freeza might have been hot enough to put Freeza down once and for all. Considering that Goku had already given him a chance to turn from his ways. Goku would have known that the fact that he still insisted on coming to earth, after all of Nameks events, shows that this was a dog that needed to be put down.
But would have put King Cold down? Goku had never met King Cold before and we can see that King Cold is deceptive enough to plea for his life. Goku's kind heart would have most likely gotten the best of him, which would have resulted mercy towards King Cold. If King Cold would have escaped, he would now have understood the opposition he was up against. He would have most likely have located Cooler, and have prepared him for what he would had to face. Having an entire family empire to control, the next step would have been a well thought out plan. Lets also consider how Vegeta was able to destroy planets from space, we can assume the same could be done by Cooler and King Cold. If they wanted, this also means that they could have destroyed the planet from space. Of course you could argue that Goku could have sensed their power from space (considering how using Instant Transmission requires one to locate the energy source no matter the location or distance), and he would intercept the earth shattering attack. Though possible, I doubt Goku would leap into space not knowing where he would end up just because he felt an evil energy signature.
In the end, I would assume that Cooler and King Cold would only make strategical strikes based on the fact they would be facing an opponent much stronger than they could handle. Which would lead to an immediate and swift attack on earth from space. If they were pride filled, then I would say they would observe the Z fighters for a certain amount of time to gauge the power levels and push themselves to match such a level either by training or acquiring techniques taken from different civilizations around the universe.
Either way, it would be bad.
When Future Trunks came into the story, he warned Goku that he and many others would die from a heart virus. Luckily he came with the vaccination and instructed Goku to take it before the virus set in. Or so we can assume.
So when Android 19 and 20 arrive on the scene, Goku exerts himself and, well what do you know, the virus becomes aggressive and Goku, in a time when the earth needed him most had to sit that fight out. And you know why? Goku decided to wait to take the vaccination up until he needed it. Why? I have no idea why anyone would think it is wise to be vaccinated for something after they already experience symptoms.
Why is this a great Mistake?
I know I have said it before, but I am going to again. Goku's kind heart. Had he been able to put such a beating on Android 19, Gero would have probably pleaded for their lives. Knowing full well that his calculations were off, there wasn't much of a choice. And I believe Goku would have given him the benefit of the doubt.
Now let's assume that this is how that played out. Gero and 19 both get back to the lab. Gero spends another couple years with this DNA sampling-observing nano bugs and now has the time to not only recalculate for Super Saiyan, but also make the proper fixes to Androids 16,17 and 18. Not to mention 19 and 20 himself will all be upgraded.
This means upon arrival of the second coming of the Androids, the Z fighters will not only not be expecting the battle, unlike how Trunk was able to warn them before. But also have to face a total of 5 fully upgraded androids. And with 16 being specialized in knowing all of Goku's moves, and upgraded to compensate for Super Saiyan, Goku would have most likely been put down fast. And all the Z fighters soon to follow.
Sure there would have never been a Perfect Cell, but the world would have ended up to be ran by the Red Ribbon Army.
If you think that Gohan seeing everyone die in front of him would have caused him to go Super Saiyan 2, as he did when Perfect Cell stepped on 16's head, I would simply counter with the fact that he didn't go Super Saiyan 2 in Future Trunks world, and that was a world where he had to watch all of his friends and mother die to the Androids. Sure Goku was dead to a heart virus in that world, but I doubt that Goku would have been the trigger of Super Saiyan 2 and not his own mother.
This is where I am going out on a limb here, but bear with me. During the Cell Games , while Goku was fighting Perfect Cell, Goku gave it his best and in the end actually went for the kill. He knew Gohan had something special, but didn't want to take the chance if he didn't have to. He went all out, and decided to use a planet busting Kamahamaha, blasting Cell in half. With that attack I am sure he struck a severe blow to Cell’s power reserve. Not to mention, Cell had to regenerate which also required some effort. With Cell in this weakened state, I propose that there was a chance that the Z fighters could have finished him off.
After realizing that he gave it his all, Goku received the Senzu bean, to recover all that was lost during the battle. He happily stepped aside and chose Gohan to take his place on the battlefield. And foolishly gave allowed Cell to have a Senzu bean. This meant that all the damage and energy loss that Cell received had now also been replenished.
Why is this a great Mistake?
If Cell didn't receive a Senzu bean, then there might have been a moment in time when all of the Z fighters realized that it was no longer time to abide by the Cell game rules and all team up against Cell. And considering the power they had available, maybe there is a chance they would have been successful. If they were to successfully obliterate 99% of Cell, he would have reformed stronger. This could have allowed him to get yet another sneak attack on some one, just like he originally did with Trunks. But this time, what if that target was Gohan? Or Goku? If the target was Goku, this means that even if Gohan went Super Saiyan 2 for some reason, Goku would not have been there to relocate a self destructing Cell. If he took out Gohan, this means that there would be a key player removed from the battle field.
Or to take another turn, Cell could have, like stated before, pleaded for his life as a deception for future revenge. Which would mean that we also would have not gotten Android 18 back and the events would have never played out for Androids 16 to get his head crushed, which means no Super Saiyan 2. This means that even if Cell as away, all of the Saiyans never knew there was a next level, so they never trained to reach it (Considering all the time they spent in the Hyperbolic Time Chamber and hadn't reached it yet).
Let's just assume that the entire team was able to actually obliterate Cell, and put him away for good. This would mean that Goku would have never died, and gone to train with King Kai yet again, and would have never learned to go Super Saiyan 3 or the Fusion Dance.
This would mean that when Babidi and Dabura arrive on earth (being that Babidi felt that Vegeta's power at that level was enough to resurrect Buu), the Z fighters may not have been able to get past Dabura or Fat Buu.
Maybe the Fusion earrings might have played a part in defeating Fat Buu, but that would have meant that Goku and Vegeta would have been permanently fused as Vegito. Considering that Vegeta is a part of Vegito, in theory creating a Spirit Bomb would have been unlikely. And if for some chance, Vegito gets absorbed, the only hope for the rest of the world would have been Mystic Gohan. And if all progresses as the normal series, making it all the way to Kid Buu, note that Goku still doesn't know about Super Saiyan 3, and in the end, everyone in the galaxy would eventually have been destroyed.
And all of this might have happened if Goku didn't give Cell a Senzu bean.
Or maybe you think that there is something I am missing? Let's talk about it.