The price of magic

he Price of Magic is the inevitable expense for using or casting magic. 
Often stated as a warning that "magic always comes with a price", but sometimes, the price is not foreseeable. Rumplestiltskin's curse has the heaviest penalty for gaining his Dark One powers. While the magic allows him to accumulate masses of power, it not only binds his loyalty to a dagger, but also corrupts and injures him both physically and emotionally. ("Desperate Souls", "Skin Deep", "The Apprentice", "Heart of Gold")
Regina casts the Dark Curse by ripping out her father's heart to enact the Curse ("Pilot").A whole is filled in Regina's heart and no matter what she does, the void in her heart will never be filled.
While being able to use the Dark magic, Rumplestiltskin is able to manipulate magic to his will ("Desperate Souls", "Skin Deep", "The Apprentice", "Heart of Gold") 
The magic ends up corrupting him with the power, even though he took the Dark One dagger to protect his son from the soldiers.
Regina has Jefferson (also known as the Mad Hatter) to be able to go back in time and use the poison apple meant to poison Emma with the Sleeping Curse ("As Blood as Red as an Apple").Besides Emma, Henry takes the apple to save his mother and undergoes the sleeping curse, backfiring on Regina's plan on getting rid of her son's biological mother.No matter what the warning, Regina does not ever listen to Rumplestiltskin.
Maddie uses a spell to make herself not look pregnant in front of her parents, her siblings and the only one who knew the secret are Doc and Grumpy. The spell causes Maddie to be in excruciating pain and causes her to see the Darkness that is within Emma Swan. It also causes an accelerated pregnancy.