Revealing mistake: I think there might be something wrong with the eels in this episode. They don't move in the tank, despite being alive and don't move when taken out either. Moreover, you cannot actually see any in the tank with one exception. Rubber replacements not up to standard obviously.
Continuity mistake: During the scene where Niles tells the chef to serve the souffle in a large bowl, there is a large bowl of food next to him with no utensil in it. When Niles says "Table Twelve is still waiting for their entree", there is suddenly a utensil sticking up in the center of the bowl.
Continuity mistake: Frasier is holding a plate of food in the kitchen, but after Roz comes in with the exploded cherries jubilee all over her, the food on his plate has changed completely. It could not have been affected by the cherries jubilee explosion as he had his back to the door when that happened.
Chosen answer: The turkey's stuffing.