Is teaching subjects outside one's specialization forbidden or permissible, given complete proficiency?

If you are proficient in teaching the subjects, and the students know that it is not your specialization, then there is no harm. However, if you are not proficient in them, or you led them to believe that it is your specialization, then it is not permissible, because it involves deceiving them. The Prophet, peace and blessings be upon him, said: "Whoever deceives is not of me."