Is it permissible to say that a believer who sees with the light of Allah can be deceived slightly, knowing that the intention is that the guided believer in a specific matter cannot be deceived in it, whereas he might be deceived in other matters in which he has not been guided?

There is no blame for a slip of the tongue saying something unintentionally, due to the Almighty's saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended. And ever is Allah Forgiving and Merciful." It is necessary to avoid obsessive thoughts and to divert the mind from them by seeking knowledge and fruitful work.