Law cannot be decided while one is sad, lazy, clowning, light-headed, or involved in wasteful things; only while one is involved in joy of a Mitzvah: Pesachim 117a
On serious issues, one should practice like a major Rabbi: Pesachim 112a
Ruling in accordance with a Rabbi who is ex-communicated: Shabbat 130b [acc. to Rashi's understanding]
One doesn't rule against the Rabbi of the Community: Pesachim 30a
The community rules in accordance with its Rabbi, even against the Majority: Shabbat 130a
Ruling against another Rabbi's prior ruling: Yevamot 108b
Ruling after becoming intoxicated from Dates: Ketuvot 10b
Ruling after becoming intoxicated by drinking a Reviit-Amount of Wine: Eruvin 64a-b; Ketuvot 10b, 13b
Ruling on Yom Tov, after the Yom Tov meal: Beitzah 4a-b; Zevachim 18a
Deciding law on a day which has a strong, distracting wind: Eruvin 65a
Deciding law requires clarity like that which one experiences when a north wind blows: Eruvin 65a
