When Mosheh Rabeinu annointed Aharon (his brother) the Kohen Gadol, there were two droplets of the annointing oil, shemen haMishchah, that would bead on the edges of his beard. (It actually describes a miracle that the two droplets would reabsorb to the root of his beard when he spoke so as not to fall off, and then redistribute into the two beads at the edge of his beard when he stopped talking.)א שִׁיר הַמַּעֲלוֹת, לְדָוִד:
הִנֵּה מַה-טּוֹב, וּמַה-נָּעִים-- שֶׁבֶת אַחִים גַּם-יָחַד.
ב כַּשֶּׁמֶן הַטּוֹב, עַל-הָרֹאשׁ--
יֹרֵד, עַל-הַזָּקָן זְקַן-אַהֲרֹן:
שֶׁיֹּרֵד, עַל-פִּי מִדּוֹתָיו.
ג כְּטַל-חֶרְמוֹן-- שֶׁיֹּרֵד, עַל-הַרְרֵי צִיּוֹן:
כִּי שָׁם צִוָּה יְהוָה, אֶת-הַבְּרָכָה--
Anyways Mosheh feared that perhaps the droplets were a sign that somehow he had misused the annointing oil. Therefore the second two passukim of the mizmor were spoken aloud by a bat kol, a divine voice, informing Mosheh that just as the dew of Mt. Hermon cannot be misused so too the oil of Aharon's beard was not misused.
Still Aharon feared that maybe the droplets signified something he had done wrong, at that moment the divine voice, the bat kol, spoke the first passuk of the mizmor, informing him that just as Mosheh was free of taint, so too was his brother who dwelt with him.
[here you can find another commentary that brings down the radak who says the mizmor is about the meeting between moshiah and the kohen gadol.]
(this is actually one of those parts of tanach that is so chock-full of secrets I can't wait till I find some Hassidut that explains it, or till I get to the Arizal on it, bezrat HaShem)