When is intercession forbidden?

Intercession (mediation), which involves using one's influence with people in authority to help fulfill people's needs, is originally permissible and even recommended, based on the Prophet's (peace be upon him) saying: "Intercede, and you will be rewarded, and Allah will decree through the tongue of His Messenger whatever He wills." It becomes forbidden if it leads to upholding falsehood or nullifying truth.