About a year ago, I wrote a post on "Come, Thou Almighty King." A few days after posting that, I transcribed the hymn and noticed an-other small feature.
The last verse begins with the lines "To Thee, great One in Three, / Eternal praises be." The second of those two lines is sung to this phrase (the tune is "Italian Hymn"):
"Praises" is sung with a melisma (Bb C Bb A), and because it's drawn out, there's a sense of those praises' being "eternal."