To save lives from being lost to probable social conflict: Eruvin 61a
To prevent use of trickery to technically avoid a transgression: Nedarim 43b, 44a-45a
A fine to prevent a sinner from being rewarded by his sin [שלא יהא חוטא נשכר]: Ketuvot 11a, 36b; Niddah 4b [For examples of this motivation as a desire of Gd, click here]
Fining people for errors, in order to prevent them from intentional violation [קנסו שוגג אטו מזיד]: Eruvin 68b, 100a-b
Making an edict which will prohibit even permitted benefit, along with the benefit from a crime [קנסינן היתירא משום איסורא]: Bava Metzia 72a
To prevent a korban from being rendered impure: Zevachim 32a
Taking certain invalid vows seriously, in interpretation and in rescension, to prevent people from using their speech loosely: Nedarim 13b, 14a, 20a
Creating an edict for the laity only, because the concern motivating the edict doesn't apply to scholars: Eruvin 40a; Nedarim 14a, 20a
According to Rav Chanan bar Rava, Rav performed a stringent practice in Damharia because the local people were lax in their mitzvah observance, and Rav sought to distance them from sin: Eruvin 6a
Rav taught a stringent rule in Afsatya because the local people were lax in their Mitzvah observance, and he sought to distance them from sin: Eruvin 100b
Purpose of the "Fences" setting midnight as a time limit on the practice of certain mitzvot: Berachot 2a
Forbidding practice of part of a mitzvah, in case people will be careless about fulfilling the complete Mitzvah: Menachot 44a
Preventing a potential error which could be born of confusion or forgetting: Eruvin 20a, 20b, 21a, 24b, 34a, 42b, 88b-89a, 89a; Rosh HaShanah 21a; Beitzah 8b; Megillah 21b, 22a; Succah 9b; Nedarim 43a; Bava Metzia 45b; Menachot 40a, 46b
Creating Institutions in case a situation may return to an earlier state, in which such an institution was required [שמא יחזור הדבר לקלקולו]: Eruvin 81a; Gittin 5a [3x], 5b, 16b
Creating an edict against an act because one might perform the act in a prohibited manner, although the act could be performed in a permitted manner: Eruvin 34b; Beitzah 8b [bottom]
An edict decreed out of concern lest the situation change, unbeknownst to the people involved: Eruvin 5a, 90b; Niddah 12b
Edicts to prevent a convert from being enticed to return to his old ways: Kiddushin 17b
A practice enacted to make people feel loss, and so aid in a worthwhile cause: Megillah 28a-b; Moed Katan 14b, 26b
An edict created in order to maintain public awareness of certain legal issues: Nedarim 44a
An edict to prevent people from taking a serious issue lightly: Menachot 52a
An edict to prevent people from errantly adopting an incorrect stringency: Menachot 52a
An edict not to engage in prohibited practices with one's children as part of educating them about the dangers, lest they become drawn to the prohibited practices: Shabbat 149b; Bava Metzia 75a
An edict to enable guardians to swear that they were not careless, so that they will feel safe handling the items in their care: Bava Metzia 83a
To avoid over-socialization with nochrim, which might lead to mimicking their practices: Eruvin 62a; Bava Metzia 70b-71a [see Rashi there]
Edicts for "Rectification of the World": Gittin 3b
An edict to prevent a seller from taking advantage of a purchaser: Eruvin 81b
Edict preventing use of a celebration-offering's child lest one raise herds of them and mis-use them: Temurah 18a
Edicts preventing people from thinking they could bring a korban asham worth less than the minimum value: Keritot 27a
Beginning Rosh HaShanah in the middle of the day, if witnesses to the new moon come at that point, as an edict to prevent people from taking that day lightly the next year: Eruvin 39a-b
Enactments to prevent people from taking the second day of holidays lightly: Eruvin 39a-b, 40a
Preventing people from forgetting a law: Eruvin 46b, 64a, 70a, 73b, 80b
Rabbinically requiring that certain gifts be given to the Kohen even where they are not required, lest people forget the protocols for giving them: Bechorot 18b
Preventing a person from forgetting to perform the second part of a mitzvah: Bava Metzia 54a
Preventing people from extrapolating an inaccurate legal lesson from an action they observe and do not understand: Keritot 24b
Preventing a lender from using a loan document in a permitted way, lest he use it in a prohibited way: Bava Metzia 72a
Forbidding a loan, lest it lead to a situation of prohibited interest: Bava Metzia 75a
Requiring a woman to make a certain declaration when she brings the first offering in a set, so that she will demonstrate understanding that other offerings must still be brought and she will not be careless about the rest: Keritot 8a-b
Preventing someone from going to mikvah because he would think himself pure, when, in fact, it might not work to purify him: Keritot 13a
Preventing people from creating a situation in which their eved will not be able to perform mitzvot: Bechorot 3a
When dealing with a talmid chacham, there is less concern for absorbing practices of nochri society: Bava Metzia 71a
To avoid creating a stumbling block: Rosh haShanah 31b [2x]; Keritot 9a
Reasons for Leniencies
Permitting an act which should, technically, be prohibited, in order to prevent a greater crime: Temurah 24b
We are more lenient regarding matters that will be handled by a court than regarding matters that will be handled by the community at large: Beitzah 5b