In the Talmud there are 24 reasons for excommunication. Many of these perhaps would be considered minor these days but perhaps not all.
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(1) insulting a learned man, even after his death;
(2) insulting a messenger of the court;
(3) calling an Israelite "slave";
(4) refusing to appear before the court at the appointed time;
(5) dealing lightly with any of the rabbinic or Mosaic precepts;
(6) refusing to abide by the decision of the court;
(7) keeping in one's possession an animal or an object that may prove injurious to others, such as a savage dog or a broken ladder;
(8) selling one's real estate to a non-Jew without assuming the responsibility for any injury that the non-Jew may cause his neighbors;
(9) testifying against one's Jewish neighbor in a non-Jewish court, through which the Jew is involved in a loss of money to which he would not have been condemned by a Jewish court;
(10) appropriation by a priest whose business is the selling of meat, of the priestly portions of all the animals for himself;
(11) violating the second day of a holiday, even though its observance is only a custom ("minhag");
(12) performing work on the afternoon of the day preceding Passover;
(13) taking the name of God in vain;
(14) causing others to profane the name of God ("ḥillul hashem");
(15) causing others to eat holy meat outside of Jerusalem;
(16) making calculations for the calendar, and establishing festivals accordingly, outside of Palestine;
(17) putting a stumbling-block in the way of the blind, that is to say, tempting one to sin;
(18) preventing the community from performing some religious act;
(19) selling forbidden ("ṭerefah") meat as permitted meat ("kasher");
(20) omission by a "shoḥeṭ." (ritual slaughterer) to show his knife to the rabbi for examination;
(21) self-abuse;
(22) engaging in business intercourse with one's divorced wife;
(23) being made the subject of scandal (in the case of a rabbi);
(24) excommunicating one unjustly
Sources: Maimonides, "Yad," Talmud Torah, vi. 14; Shulḥan 'Aruk, Yoreh De'ah, 334, 43.