For a franchise full of master arts mentors and masters, one of Dragon Ball's most reputed tactical minds goes to the stoic Namekian warrior Piccolo. Initially introduced as the original Dragon Ball series' final antagonist, Piccolo became a firm friend and ally to the Z Fighters over the course of Dragon Ball Z and has lent his strategic expertise to the younger generation of heroes throughout the franchise. However, for all of Piccolo's stern combat wisdom, he has repeatedly made the same mistake of letting some of his most dangerous opponents live to fight another day, often resulting in the Z Fighters paying dearly for this lapse in battlefield judgment.
Originally, Piccolo was just as merciless as his evil father, the Demon King Piccolo, finishing off any villain that he defeated, including the mortally wounded Raditz at the start of DBZ and Garlic Jr.'s henchman in the 1989 anime film Dragon Ball Z: Dead Zone. However, by the Android Saga, years into seeking redemption and fighting alongside the Z Fighters rather than against them, Piccolo's first instinct was no longer to execute his fallen opponents and he became more easily distracted. This first became especially apparent when Piccolo battled against Doctor Gero, with Piccolo's attention disrupted by the arrival of Future Trunks, allowing Gero to make his escape.
Piccolo allowing Gero to escape not only allowed Androids 17 and 18 to become reactivated and terrorize the Z Fighters but would be an error that carried over to his battle against Cell. When Cell and Piccolo first faced off, though Cell was a formidable enemy after absorbing the life energy of thousands of innocents. But, Piccolo was far stronger after recently fusing with his other half Kami to become a Super Namekian. As Piccolo was poised to destroy Cell, the synthetic villain was able to temporarily blind Piccolo and escape using the Solar Flare technique, a move that Piccolo was already acquainted with but failed to react in time accordingly for. This set up Cell reaching his perfect form and eventually inflicting casualties on the Z Fighters during the Cell Games Saga.
This trend would continue into the Dragon Ball Super manga series, with Piccolo among the Z Fighters defending Earth from the invading Galactic Prisoners working for the villainous Moro. After defeating the escaped convicts, Piccolo decided to spare them, so long as they immediately left the planet behind. This resulted in the Galactic Prisoners returning to report to Moro, informing their evil boss of the powerful fighters present on Earth. With Moro hungry to absorb their power, this led to the villain personally leading a renewed invasion attempt, with the fate of the planet hanging in the balance from Piccolo's mercy allowing his enemies to name Earth a major target for Moro.
Piccolo is certainly not the only Z Fighter who has spared their opponents only for them to return as an even greater threat than before, with Vegeta purposefully allowing Cell to reach his perfect form perhaps remaining as the Z Fighters' most heinous error. Although if it weren't for mercy, characters like Piccolo, Vegeta, and Majin Buu would have never survived to become Z Fighters themselves in the first place. However, whenever Piccolo makes this particular mistake it does seem to result in disastrous consequences for the heroes, from the reactivation of the androids to Moro attacking Earth directly, in a repeated lapse of judgment from Dragon Ball's mightiest Namekian.