Rashi on Leviticus 4:13/רש"י על ויקרא ד׳:י״ג

1עדת ישראל [AND IF THE WHOLE] CONGREGATION OF ISRAEL [ERR] — This is the Great Sanhedrin (Sifra, Vayikra Dibbura d'Chovah, Section 4 2; cf. Horayot 4b).

2ונעלם דבר AND A THING (a matter of law) WAS HID [FROM THE EYES OF THE ASSEMBLY] — i. e. they (the Synhedrion) erred, deciding in respect to one of all those acts for which excision is, according to tradition), implicitly mentioned in the Torah that it is permissible (Horayot 8a).

3הקהל ועשו — [AND THE THING BE HID FROM THE EYES OF] THE ASSEMBLY AND THEY HAVE DONE — It means that the assembly did according to their (the Sanhedrin’s) decision (cf. Horayot 2a).