Before Stimpy looks at the mirror, he has pajamas on. When he does, his pajamas are gone.
Ren's black tooth switches places from left to right.
In "Big House Blues", when Ren says, "Full of shelter", his mouth doesn't move.
When Stimpy hands Ren the mirror, his legs are missing.
When Ren groans about his toothache, his mouth does not movie.
When Ren says, "Why, Stimpy? Why?", his mouth does not movie when he says "Why?"
When Ren wakes up Stimpy, Stimpy is not wearing his pajamas.
There are 3 scenes where Stimpy's collar is missing.
Despite the fact that the title is called "Ren's Toothache", Ren only had a toothache one time.