How do we differentiate between a scholar's lapse and an innovator's heresy?

A slip (zallah) is an error. It can be an innovation (bid'ah) or an error that is not considered an innovation. They can be distinguished by:

1. The one who slips: One who sincerely follows the methodology of the Salaf but errs due to a deficiency in understanding or knowledge is not considered an innovator. However, one who follows his desires in deriving rulings, pursuing the ambiguous (mutashabih) and abandoning the clear (muhkam), is the innovator. 2. The nature of the evidence: Innovation relies on what is not a valid proof, such as a fabricated hadith, or intellectual preference (istihsan aqli) in matters of Sacred Law, or reliance on emotions and dreams, or reinterpreting sound texts to conform to desires. 3. The domain of the slip: A non-innovative slip occurs in areas of legal ijtihad (such as transactions), where a scholar may be excused due to ignorance or interpretation. As for the slip of innovation, it relates to aspects of Sacred Law that have already been definitively established for us, such as introducing new acts of worship or altering well-established legal rulings. This is what Al-Shafi'i and Ibn Al-Qayyim warned against.

The fundamental difference between them is that a scholar strives for the truth and may err, in which case he is excused, whereas people of desires follow conjecture and whims and assert what they have not been commanded to do.