When Stimpy asks Ren to try out the Happy Helmet the inside of Ren's ear turns the same color as the couch.
Stimpy says that the "Happy Helmet" is controlled by a dial on a remote. However, he used the remote as if it were a button, and pressed it instead of turning it like a dial.
In three scenes, Stimpy's nose is colored red.
In "Stimpy's Invention," Stimpy's eyes temporarily move off of his head for a few frames when he turns to Ren to say "They're full of glue."