It has been narrated from Imam Ja’far al-Sadiq (‘a) that:
كان المغيرة بن سعيد يتعمد الكذب على أبي، و يأخذ كتب أصحابه و كان اصحابه المستترون بأصحاب ابي يأخذون الكتب من أصحاب أبي فيدفعونها الى المغيرة، فكان يدس فيها الكفر و الزندقة، و يسندها الى ابي ثم يدفعها الى اصحابه و يأمرهم ان يبثوها في الشيعة، فكلما كان في كتب اصحاب أبي من الغلو فذاك ما دسه المغيرة ابن سعيد في كتبهم
al-Mughira ibn Sa’eed used to intentionally lie about my father. He would take the writings of his companions, and his followers would hide themselves among the companions of my father, taking what had been written by my father’s companions and passing it to al-Mughira who would then interpolate the text and add into it [teachings of] disbelief and apostasy and attribute it to my father. He would then give the books back to his companions and instruct them to disseminate them among the Shi’a. So everything in the books of my father’s companions that contains elements of extremism (ghuluw) is actually from the interpolations of al-Mughira ibn Sa’eed that have been introduced in their books.
[Rijal al-Kashshi 2:491]
Upon examination of this interesting narration, we find that the Imam himself describes in detail how the enemies would go to great lengths to corrupt the teachings of the Ahl al-Bayt (‘a). The interesting point to note though, is that we are told that whatever we find in the books of the Imam’s companions that contains elements of ghuluw should be taken to be interpolations and not ascribed to the Imam. Now the question arises – what exactly is this ghuluw and how can we draw the line that marks the boundary between ghuluw and fadha’il, for example?