During "Everybody Wash", Jim Henson's arm can be seen when Ernie says "everybody wash...your everything!"
During the scene with Ernie in the tub, we can see Jim Henson's arm operating him, as well as Frank Oz's arm (plus his head and black-rimmed glasses) operating Bert.
During Kermit's lecture on the letter W, Frank Oz's arm can be seen just below Cookie Monster while he is nibbling on Kermit.