Personal Gain

Paige is "busted" for using her powers for personal gain

Personal gain is the term applied when a good magical being (such as a witch) uses their powers for their own profit. Magical beings that do so risk suffering certain consequences; ranging from simply losing any benefit gained to someone being killed. Hence, when spells are cast for personal gain, they could backfire.

The Wiccan Rede

"An it harm none, do what ye will."

The Wiccan Rede is the most basic yet fundamental of magical rules, which establishes that good witches cannot use their powers for their gain, including beauty and love spells. They are also not allowed to use magic for punishment purposes even if the person is truly guilty of an offense, for good witches are only supposed to "protect the innocent, not punish the guilty."

If a good witch goes against the Wiccan Rede, he or she will have to face consequences: he or she would have to either suffer punishment, or the magic that he or she had used would backfire on him or her. An example of this could be seen when Paige used magic to punish a man who had a habit of making sexist comments, but it resulted in her having enlarged breasts.

It is also possible for witches to not suffer from any negative consequences, but instead, any benefits from their acts of personal gain would be negated. An example of this could be seen when Phoebe tries to win the lottery after having a premonition of the results, only for the corresponding numbers to disappear from the ticket while the results are being broadcast.

Since evil is already in violation of the Wiccan Rede and therefore not bound by any issues of morality, they are not limited by personal gain and could use magic however they desired. Phoebe herself pointed out that this was what made evil so tempting - the sensation of being able to do whatever they wanted whenever they wanted, without the voice of conscience holding them back with reminders of morality and the potential consequences that could stem from their actions. However, Paige also pointed out that this was what separated Good from Evil.

Neutral Beings

Personal Gain also poses a threat to neutral beings such as the Avatars. Alpha attempted to dissuade Cole Turner when he used his new powers to create a alternate reality in a last ditch to get Phoebe back. In this reality he never got sent to the demonic wasteland, thus becoming vulnerable and finally being vanquished. When Leo became an Avatar, he used the power of Chronokinesis unintentionally and was warned by Alpha not to do so again. However, personal gain for Avatars is strongly discouraged because it weakens the collective magical power rather than the rule itself.

Mortals Gaining Magic

The result of mortals using magic

After the Darklighter, Rennek used stolen magical knowledge from Leo, he caused magic to switch sides to all beings that were on Earth at the moment. Because of this, mortals gained the ability to use magic.

With the Earth cut off from the Elders, the Underworld, the Angels of Destiny, and all forms of magical authority, mortals used magic for their selfish benefits such as magically altering their houses or making their cars fly without regard. Piper believed that there would be repercussions, though it is not known if personal gain applied to mortals, as they were technically a neutral party. Eventually, people started forming gangs for criminal activities, and military squads to repress them and capture former magical beings for studies.

After the natural order of magic was restored, Prue Halliwell cast a worldwide memory-wipe spell to protect its secret once more, though also ensured that the mortals would never fully forget either.

Notes and Trivia

• The rules of personal gain were featured more in earlier seasons than in the latter, where the sisters started using magic more casually and it often went unpunished. Some cases could be viewed as bordering on harming mortals, such as Phoebe using a potion to turn a rival advice columnist into an animal on more than one occasion. A notable exception is "Crimes and Witch-Demeanors", where Phoebe Halliwell loses her powers after using them extensively for personal gain. 
• There are small instances of personal gain throughout the series with no punishment to follow up, mostly in the form of Piper freezing annoyances or Prue (and later Paige) telekinetically moving objects to complete mundane tasks. They might not have warranted any consequences since they were only minor offenses with little benefit. 
• Phoebe once tried to avoid the consequences of personal gain by adding "PS, there will be no personal gain" at the end of her Smart Spell. Piper later advised her to word her To Hear Your Heart's Desire spell in a manner that avoids personal gain.
• In "Piper's Place", Wyatt uses the Charm of Multiplicity to create several Pipers. More clones appear as time passes until Wyatt reverses the spell. This is presumably a side effect of personal gain, or Wyatt overloading the spell with his magic.

Examples

• Phoebe's lottery ticket turning blank after attempting to use her power for profit.
• Prue cast the Truth Spell and she and her sisters retained their memories after it ended while everyone else forgot.
• Piper cast the Charm of Confidence with a double dose and caused a waiter to turn into a womanizer and completely ignored his ex-girlfriend.
• Paige caused her breasts to enlarge after casting a spell to punish a pervert.
• Paige cast a Vanishing Spell to help a woman win custody of her son at the cost of her co-worker's promotion.
• Piper cast a Fearless Spell that suppressed her fear to the point of acting recklessly and nearly got her killed while facing Necron.
• Piper cast a Memory Spell to help her and Leo remember their happy and sad times together. It accidentally transported Phoebe, Paige, and a Familiar Hunter through every period they recalled at the moment.
• Chris made David Right charm all three sisters into agreeing to bind Wyatt's powers, unwittingly pitting them against each other.
• Phoebe accidentally cast a spell that turned her back into her rebellious teenage self with all of her powers intact.
• Paige cast the spell To Create a Perfect Man for her needs while caring for her sisters. While it created a man befitting of her image, it also created an evil counterpart who aimed to turn her evil by appealing to her dark side.
• Grams cast a spell to Resolve Sibling Rivalry which extracted the rivalry from her great-grandchildren and transferred to her granddaughters instead. They reverted to child-like behavior until Patty reversed the spell.
• Wyatt cast the Charm of Multiplicity to create several clones of Piper. As time went by, more Piper clones appeared and weakened the real Piper until Wyatt reversed the spell.