Chabad.org — English translation + Rashi
Torah Temimah points to Horayos 4a (unit 5; single primary locus per emergency Core plan).
E-Daf.com — Horayos 4a
HebrewBooks / Shas PDF
English: “They err and it is hidden” — this teaches that they are liable only for a forgotten law together with an erroneous act.
Dual condition for communal chatas — communal liability requires both a forgotten ruling and an actual prohibited act.
The congregation becomes liable only when two elements coincide: the Sanhedrin’s forgetting of a law and the people’s performance of a forbidden act on the strength of that forgetting. A pure intellectual error without action, or an action without a prior forgotten ruling, does not trigger the offering.