Recently when watching the celebrity version of "Monster in the Mirror", when the various celebrities first appear and all sing together, we actually hear the bridge of the song sung by Grover and the monsters, as heard in audio recordings of the song (but not in the broadcast version of the non-celebrity version). I never noticed this before, but considering that the celebrities were all tapped seperately, I guess this is an easy way for all the singing to sound simultaneous.
Also, when watching "Put Down the Duckie", I noticed that Hoots actually doesn't wear his vest. All this time I thought he did. I'll have to check and see if he wore it in "Follow That Bird" and "Wake Up/Goodnight".
Also, I noticed that in the non-celebrity version of "Put Down the Duckie", at the beginning he says "Come in, Ernie!", while in the celebrity version he says "Come in, Ernie, my man!". Watching the scenes closely, they are actually different takes (I had to check and see if maybe the "my man" part was dubbed).