The first stanza of the popular Friday-night song "שלום עליכם" calls angels "מלאכי השלום מלאכי השרת מלאכי עליון" (or, in other versions, just "מלאכי השרת מלאכי עליון"). The other stanzas call them merely "מלאכי השלום מלאכי עליון". Why the change? That is, why does the first stanza and no other call the angels "מלאכי השרת"?
Rabbi Shlomo ben Eliyahu suggests that based on the Chazal that on Friday night 2 angels escort everyone home, and then if the table is set and prepared the good angel blesses the household that it should be the same in the coming week. The bad angel is forced to respond Amen. Therefore at first they are Malachei Hasharais and then when the 2 angels make peace they are Malachei Hashalom.
Rabbi Yonasan Wiener asked this question of the Lubavitcher Rebbe when he was nine years old. This is his recollection of the answer:
The Rebbe said, “If you notice, the first two stanzas seem to be redundant. The first one says, ‘Peace unto you, ministering angels,’ and the second one says, ‘May your coming be in peace, angels of peace.’ It seems that both are greetings of welcome, so why do we need to welcome them twice?”
He went on, “I’ll tell you the reason. There are two types of angels – weekday angels and Shabbos angels. The ‘ministering angels’ are the weekday angels of servitude, and we are not welcoming them, but saying good-bye to them. In Hebrew, hello and good-bye are the same word, Shalom, so in the first stanza we are really sending off these weekday angels because Shabbos has begun. After that, we are greeting the Shabbos angels, the ‘angels of peace.’ So none of this is redundant or superfluous.”