Wishing good Shabbos after the time of Mincha:
There is a known custom not to wish good Shabbos or say Shalom Aleichem between Mincha and Maariv on Shabbos, during Bein Hashmashos.
 Piskeiy Teshuvos 292/8 in name of Sefer Matamim and Otzer Minhagei Yeshurun; Mentioned in Makor Chaim 270
Chabad custom: Rabbi Leibal Groner related to me in a correspondence that the Rebbe followed this custom and did not wish good Shabbos between Mincha and Maariv of Shabbos day.
 The reason: This is because Moshe Rabbeinu, Yosef and David each passed away during Mincha time of Shabbos, and hence in mourning of their death we do not say Shalom to each other. [ibid; See Admur 292/5-6]