In "Everyone Loves Uncle Bob", when the kids are chanting "We want Uncle Bob!", you see a full picture of all the kids. Look to the direct right of the camera, the only kid that isn't chanting, is Mandy from The Grim adventures of Billy and Mandy.
Uncle Bob: (about Hector infiltrating the studio) I don't remember this in the script. Do you, Warren?
(Warren nods no)
Hector: That's because it's not in the script, Uncle Boob! For I, Hector Con Carne, am your mid-season replacement.
Even though Major Dr. Ghastly appeared in "Everyone Loves Uncle Bob", she had no dialogue.