A March 13, 2010 Wall Street Journal headline offered a nice play on "apple": "Forbidden Fruit: Microsoft Workers Hide Their iPhones."
Forbidden fruit names a pleasure that's immoral, illegal, or both.
In Genesis, God forbade Adam and Eve to eat the fruit of the Tree of Knowledge of Good and Evil. The serpent tempted them, they couldn't resist, so God exiled them from paradise. And here we are today.
What was the fruit? The usual suspect is apple. In Latin, malus means both evil and apple tree. Other contenders include grapes, figs, quinces, and pomegranates.