Serving as the eponymous Deus ex machina of the series, Earth’s Dragon Balls have been sought after for so many various reasons. From a desire for immortality to merely wanting to bring a friend back, the Dragon Balls seem to have so many uses.
Yet even Dragon Balls have a limit which they reach. Specifically, Dragon Balls (of Earth’s or any other kind) cannot grant any wish which exceeds the power of their creator. Earth’s set of Dragon Balls is no exception to this rule, whether discussing Kami’s set or Dende’s set.
Since they exceed the power of their creator, these eight wishes can’t be granted using Earth’s Dragon Balls
1. Reviving someone who died of natural causes
While this may seem a strange wish to be unable to grant, it makes sense considering Kami’s role on Earth as well as the Balls’. To interfere with the course of life and nature would be above the power Kami wields, even serving as a quasi-God of Earth. This rule even adds drama to Dragon Ball Z at one point, with Goku at severe risk of dying from a heart attack and thus not being able to come back.
2. Granting the same wish more than once
While an exception is made here to bring people back to life (still with a limit of two times), most wishes made via the Balls can’t be wished for twice. Krillin being revived twice via Earth's balls is a perfect example of revivals trumping this limit. Otherwise, this is likely to prevent someone from amassing wealth simply by locating the Balls for a year, then wishing oneself a year’s salary for the task.
3. Negatively impacting enemies who exceed the creator’s power
Regardless of what’s requested to be done, Earth’s set of Balls are unable to harm anyone whose power exceeds that of the set’s creator. From a literary and practical standpoint, this rule makes sense; being able to wish any enemy goodbye would remove all tension from the story, and how is someone supposed to hurt one who’s stronger than them?
4. Completely restoring a biotechnical being to normal
As seen in Dragon Ball Z’s Cell Games saga, Shenron is unable to completely turn a mechanical being like an android to its original humanity. This is a somewhat interesting limit, as Shenron is able to affect life and death but not mechanics. Nevertheless, it is a confirmed limitation of Earth’s current set of Balls.
5. Wish limit
Whether one, two, or three, each and every set of Dragon Balls (including Earth’s) has some sort of wish limit. In addition to the spreading of the Balls after each use, this is likely to prevent someone from abusing the system via wishing for limitless wishes. It also makes sense from a literary standpoint to put a hard use limit on a story’s deus ex machina.
6. Reviving people who’ve been dead for more than a year
While this only applies to Kami’s original set of Balls, it used to be impossible to wish back someone who’s been dead for more than a year. Frieza’s resurrection would seem to indicate Dende’s set eliminates this rule; however, this has yet to be officially addressed one way or another.
7. Affecting cooldown period
Whether it's a Namekian or Earth set, the Balls all have a cooldown period after usage which can’t be affected. For the Earth set, they take a full year before becoming usable again. Namek’s set, however, only requires four months due to the planet’s three suns. In either case, these cooldown periods are absolute and cannot be affected
8. Wishing for the destruction of the Dragon Balls
Although there are many ways around this, one can’t use the Balls to wish for the destruction of the Balls. While this rule certainly does prevent Shenron from destroying himself and the Balls, one can kill Shenron or the Ball’s creators instead. The former was the strategy King Piccolo used in order to ensure no one could use the Balls after him.