The Gemara here distinguishes between two types of mistakes: (a) a "mistake concerning the words of the Mishnah," and (b) a mistake concerning "balance of judgement".

The first type of mistake is where the judge overlooked an explicit Mishnah. The Gemara here adds that this does not mean necessarily a Mishnah, because overlooking a Tosefta, a discussion of Rav and Shmuel, or even of Ravina and Rav Ashi (who lived at the very end of the Talmudic period) is also considered as overlooking something explicit.

A mistake concerning "judgement," or "Shikul ha'Da'as," is where there is a dispute between Tana'im or Amora'im, and no explicit ruling was given, but the majority of judges side with one opinion. The judge in this particular case ruled acording to the minority opinion. Possibly nowadays this would be similar to a Halachic issue where it is known that the majority of the major Halachic authorities rule one way, but the Dayan in the Beis Din ruled according to a minority opinion without any special extenuating circumstances existing to justify this.

Kol Tuv,

Dovid Bloom