76:1 The custom is to say Maariv earlier on Friday nights than during the week in order to start Shabbos as early as possible, so long as it's after plag hamincha (i.e., 1 ¼ “seasonal hours” before sunset or before night – see OC 263:4 and Mishnah Brurah 263:19.). Even those who are normally careful to recite Maariv on weekdays only after the stars appear may say it earlier on Shabbos. Even though Mincha during the week is sometimes said at the same time that one now says Maariv for Shabbos, this does not present a problem when we are presented with the opportunity to extend Shabbos into the weekday. (One davening by himself should preferably wait until the normal time for Maariv – Bi’ur Halacha 267:1, s.v. im hatzibbur.)
76:2 The bracha “Hashkiveinu” (“cause us to lie down”) has a different ending on Shabbos than it does during the week. It does not end with “Shomeir amo Yisroel” (“Who protects His people, Israel”), because this is prayer for protection. This is not necessary on Shabbos, which is itself a form of protection. Instead, we say “U’fros aleinu succas shlomecha” (“spread over us the shelter of Your peace”); we also recite this conclusion on yom tov. If a person accidentally recited the weekday version on Shabbos, if he realized his error immediately after saying the last word of the bracha, he should then say the proper Shabbos conclusion without delay. If he only realized his error after a slight delay, he need not do anything further.