1) Ch. 18, v. 2: "Shloshoh anoshim" Three men Rashi says that one of these three angels was sent to heal Avrohom. The gemara B.B. 16b quotes Rabbi Shimon bar Yochai, who says that Avrohom had a special stone that he wore on a necklace, which brought healing to anyone who would look at it. If so, why didn't he simply look at the stone?
2) Ch. 18, v. 8: "Va'yikach chemoh v'cholov u'ven habokor" And he took butter and milk and the young calf The gemara Yoma 38b says that Avrohom fulfilled the commandments of the Torah even before it was given, and even "eiruvei tavshilin." The question raised on this is why "eiruvei tavshilin" is singled out from among all the mitzvos.
3) Ch. 18, v. 21: "Eirdoh noh v'er'eh" I will now descend and I will see Rashi derives from this that when a judge is involved in ruling a possible death penalty, he must see the facts himself. This is most puzzling, as a judge may accept the words of witnesses and does not have to be an eye-witness.
4) Ch. 19, v. 25: "Va'yahafoch es he'orim ho'eil" And He overturned these cities We know that Hashem destroyed these communities by raining down fire, sulfur, and salt. Why was it necessary to also plow these cities under?
5) Ch. 20, v. 15: "Hi'nei artzi l'fo'necho batov b'ei'necho sheiv" Behold my land is available to you in the area that is best in your eyes reside Contrast this appeasing offer with Paroh's abrupt send-off, "Hi'nei ish't'cho kach vo'leich" Here is your wife take her and go (Breishis 12:19). Why the difference in behaviour?