At the end of "I Hope That Somethin' Better Comes Along", when Kermit the Frog and Rowlf harmonize on a final pair of long notes, they both hold the notes, audibly, until the end of the song, but Rowlf's mouth changes to a scat/improv line that he sings and we hear. (In other words, his voice is heard on two different musical lines at the same time).
Immediately after Kermit the Frog and Fozzie's first encounter with Doc Hopper, as Fozzie is driving away, Doc Hopper speaks several lines ("Hey, that's my car!", "I'm offering you money!"), but his mouth does not move.