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.
First Day of High School - S3-E10
Continuity mistake: In Season 3, "First Day of High School," when the principle is telling London that she cannot build her lounge, the position of her hands change from being on her hips to being in the air.
Revealing mistake: When Cody falls through the air vents, you can obviously see that it's a dummy. When "he" falls, his body is perfectly straight. If you really were falling, you'd be screaming and moving.
Visible crew/equipment: When Chef Pablo shows them the greatest food, you can see the studio light and camera reflecting off the domes.
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.