As much as Jim innovated the idea of lip-syncing by keeping the four fingers as still as possible and moving just the thumb, I've come to notice there seems to be a period in the early 70s where his work is a little off and the puppets he performers move their skulls a little too much when talking:
Of course, it could very well be chalked up to the particular puppets he's using; speaking from experience, I can tell you if a puppet's mouth isn't hinged very well or if the whole head might be a little constricting that can affect the head and mouth movements and sometimes causes the mouth to open in a way that makes the head look as if it's flopping around too much.
Conversely, however, I've always found this to be one of Jim's perfect examples of lip syncing:
Not only does he hit every syllable precisely, but notice "fire" is just one syllable, whereas most Muppets seem to use two.