The Arwin That Came To Dinner - S3-E8
Factual error: In Season 3, "The Arwin That Came To Dinner," in the scene where Arwin is licking his plate clean (that supposedly had food on it), notice that when he does this you can hear that high pitched waxy sound. Oddly enough, if you get a clean plate straight from the dishwasher and rub skin against it, you can hear that same sound. Meaning that there was never any food on it.
Answer: Keep in mind, Muriel wasn't very popular. So the writers/directors must have thought it'd be a good idea to let go of the character. No one seems to be complaining.