In season 3, episode 26, Fran and the family take a cruise and she's wearing a very revealing bikini - no mention made of the famous tattoo that she won't reveal in a bikini now.
In the kitchen, Sylvia has a chocolate soda. In one shot it's light colored, the next dark colored, then the last shot as Niles hands it to her it's light colored again.
Fran sits on a doughnut at the end of the show and quips that all donuts "end up on my hips". One does not sit on their hips, but their rear. This is a common error in USA. The hips are, of course, located on either side of the body.