It is reported that Mufaddal b. Muhalhal – Allāh have mercy on him – said:
If the heretic started mentioning his bid’ah right at the beginning of the sitting, you would be on your guard and flee from him, but what he does is begin by mentioning hadīth from the Sunnah. Then, he slips his bid’ah in on you. It might then stick to your heart; but when will it leave your heart?

Ibn Battah, Al-Ibānah Al-Kubrā, 2:444.