Dragon Ball's lead character Goku isn't dumb, but he's made more than a few unwise decisions!
From letting Frieza reach 100% of his power to turning a driver's test into a race, to other things besides, Goku has not always thought things through. It's not like he doesn't think when it comes to fighting either.
Note: The following contains spoilers and is only the author's opinion.
Goku's ten most unwise decisions in 'Dragon Ball'
1) Letting Frieza reach 100% power and then giving him energy afterward
During Dragon Ball Z's Frieza saga, Frieza completely dominated and devastated everyone and everything that came in his path. He killed Krillin and Vegeta, a ton of Namikians, and very nearly got Gohan and Piccolo too.
Despite this, when Goku went Super Saiyan and was tearing Frieza a new one, he let the villain reach his full power to settle things in a fair fight.
Not exactly smart, but then again, neither was giving him energy afterward. This immediately backfired on Frieza after he tried betraying Goku because the latter blasted him to bits.
Admittedly, Frieza survived this and the planetary explosion, but he'd get his comeuppance time and again.
2) Turning a driver's test into a race
A simpler one from one of the best filler episodes Dragon Ball Z ever had, Goku and Piccolo were chewed out by Chi-Chi on getting a car. She was tired of walking all the way home, and having to cook and run a bath for her family was her last straw!
She demanded they get a car, or they'd have to cook their own meals.
Goku then insisted on turning what should've been a routine driver test into a high-speed race that failed them outright, plus he couldn't even drive properly. The other, better, part of the episode was when he tried to save a bus from going over a cliff by ramming into it and when that failed, he and Piccolo had to rescue the bus.
3) Letting go of Raditz's tail
In Dragon Ball Z's Saiyan Saga, Raditz kidnapped Gohan and forced Piccolo and Goku to bury the hatchet and fight him. Goku had Raditz by the tail, effectively pinning him down and leaving him wide open for the Special Beam Cannon.
Raditz begged Goku to let go, and after he did so, Raditz proceeded to beat Goku down. It took sacrificing himself to finally get rid of the villain.
Goku may be kindhearted, but there should be a limit to when kindheartedness crosses over into stupidity.
4) Not explaining his plan with Cell
Giving the actual Senzu Bean to Cell during Dragon Ball Z's Cell saga wasn't the problem. It may not have been too bright, but Goku realized he couldn't beat Cell. Gohan could.
The problem is that he didn't explain himself adequately in the anime or the manga. Mind you, this is Goku. Since when has he ever explained things properly?
5) Letting his guard down around Moro
Speaking of letting his guard down, the Dragon Ball Super manga shows Goku doing something rather stupid. When he's in the middle of fighting the planet parasite Moro, he powers down to question whether or not he'll go back to jail.
He gave him a senzu bean, which Moro then ate and tried to hit Goku again but broke his hand on Goku's Ultra Instinct body.
The dumb thing isn't luring Moro into that trap, it's not finishing him when he had the chance. It allowed Vegeta to come in and help with the Spirit Fission and Moro eventually overloaded himself, but Goku could've finished Moro earlier and avoided the headache!
Granted, there's room for interpretation as Goku does believe in giving people chances, and Moro proved he didn't deserve it. Goku isn't one to kill at the first shot.
6) His rather dumb plans with Buu
Majin Buu is pretty powerful in his own right. Goku went to Super Saiyan 3, burned through most of his life force, and tried to beat him. He didn't exactly finish the job and had to come back to do it with Super Buu.
The fusion with Gohan didn't work. Vegeta was prideful enough not to try again, and unfortunately, the breaking of the earrings meant that they couldn't finish Super or Kid Buu off.
There were plenty of opportunities to finish Buu off that Goku didn't take.
7) No time for family, only training!
This one is a more personal, irksome quality Goku has. When Vegeta is a better parent than Goku (proven in Dragon Ball Super), and even Krillin, you know Goku messed up.
He wasn't there for Goten's birth when Vegeta stopped training for Bulla's birth before Dragon Ball Super's Tournament of Power. He never held down a job, unlike Krillin and Android 17. Goku seems more comfortable being dead than alive, neglecting his family.
This point may be controversial, but it's not like Goku doesn't love his family. It's just that he rarely spends actual time with them. Even trying to train Uub was a faux pas.
8) Letting the Androids get built
This takes priority over the heart virus medication not being taken, as the heart virus struck Goku much later in the timeline than Trunks warned. In the Android Saga of Dragon Ball Z, Bulma suggests looking for the Androids or at least starting the search. Goku declines due to wanting to fight them.
So, let's count the things that might've been prevented if they took that opportunity: 19 and 20 wouldn't have existed, 16, 17, and 18 wouldn't have come, and maybe they could've beaten Cell early.
The other possibility was that 19 and 20 came early and killed them all due to lack of training. Or worse, 17 and 18 activated early.
"Always in motion, is the future" are good words from Yoda to keep in mind. Futures aren't set in stone.
9) Getting the wrong Evil Containment wave tag
This one comes courtesy of the Dragon Ball Super manga. The long and short is that Goku, Future Trunks, and Vegeta are facing off against the fused Zamasu and losing horribly.
So, they came up with a plan courtesy of Master Roshi: use the Evil Containment Wave/Mafuba to seal him away.
This would've worked had Goku not gotten the wrong tag. Roshi shares some blame for this one, even though he did warn Goku.
10) Moving too quickly to avenge Krillin
When Krillin originally died for the first time (and not the last) in the original Dragon Ball, Goku charged after his killer Tambourine with all the rage and fury one would expect. The problem is, he did this rather rashly after just having fought Tien, so he was very weak.
This caused Tambourine to smack him around and destroy the original Flying Nimbus. Goku would, after some rest time, kill Tambourine with a Kamehameha.