I was trying to find the prayer birkat haMazon (grace after meals) online because I thought it might be easier to print it out instead of copying it. After coming up with the right search words (the first phrase of the prayer in Hebrew), I quickly found the vocalized text and also the transliterations and tranlations I wanted (courtesy of Chabad, mostly).
But when I took a closer look, I was very surprised to see that there were a few verses missing from what I know and what I have in my printed bentcher (prayer book for grace after meal). On Shabat, after Psalm 126, there are the following four verses added:
תְּהִלַּת יי יְדַבֶּר פִּי וִיבָרֵךְ כָּל בָּשָׂר שֵׁם קָדְשׁוֹ לְעוֹלָם וָעֶד:
My mouth shall speak the praise of the LORD; and let all flesh bless His holy name for ever and ever. (Psalm 145:21)
וַאֲנַחְנוּ נְבָרֵךְ יָהּ מֵעַתָּה וְעַד עוֹלָם הַלְלוּיָהּ:
But we will bless the LORD from this time forth and for ever. Hallelujah. (Psalm 115:18)
הוֹדוּ לַיי כִּי טוֹב. כִּי לְעוֹלָם חַסְדּוֹ:
O give thanks unto the LORD, for He is good, for His mercy endureth for ever. (Psalm 107:1)
מִי יְמַלֵּל, גְּבוּרוֹת יי, יַשְׁמִיעַ, כָּל תְּהִלָּתוֹ:
Who can express the mighty acts of the LORD, or make all His praise to be heard? (Psalm 106:2)
Apparently this is a German custom and the verses allude to the four blessings the birkat haMazon consists of (Rabbi Rackman: The Custom Of Adding Verses In The “Bentching”). Interesting!