Continuity mistake: It is known that Dorothy married Stan because she was pregnant and they were married for 38 years. However, we know Michael is not 38 (in a couple of episodes they state his age as either in his 20's or 30) and in the 2 episodes her daughter is in, she doesn't look to be anywhere near 38 either.
Continuity mistake: In one episode, Rose talks about being valedictorian of her high school class in St. Olaf. In another episode, Rose goes to night school (where Dorothy is her teacher) in order to get her GED because she never graduated from high school. While talking to rose about the class Dorothy states the class is for people who didn't get a diploma. Additionally, in the episode "Whose face is this, anyway" Rose mentions that she was in a sorority called the "Alpha Yams"; when the girls look at her strangely, Rose states "It was an agricultural college." She could not have gone to college without a high school diploma.
Other mistake: In the opening scene showing the outside of the house, the garage and driveway are on the left side of the house. In "End of the Curse," the girls raise minks in the garage, which is accessed from the back corner of the kitchen, thus the garage is actually on the opposite side of the house. Furthermore there are four large bedrooms and at least one large bathroom down the hallway. The hallway shoots off the living room at an angle. There is not enough room to fit all those rooms between the supposed garage and the kitchen and the lanai; all of the rooms would overlap one another. Lastly, the lanai seems to be surrounded on three sides by the house (as is typical of a lanai) but the hallway leading to the lanai, and the way the girls enter from the left, would put it on the very front corner, and surrounded by nothing.
Other mistake: In the episode where Blanche and Rose became big sisters to two girls there is a mistake the girls make the first time they are in the house. Blanche says she is going to show them make up tricks in her bed room and the two girls go to her bedroom without asking where it is or which one it was while Blanche was still talking.
Continuity mistake: In the episode when Dorothy is trying out for Jeopardy, she is sitting on the couch studying, When Sophia sits down to talk to her Dorothy is looking at a page with just words, It cuts to Sophia and then to Dorothy and the page now has pictures on it. You can still see the corner of the book when the frame is on Sophia and you can see that Dorothy did not turn the page.
Character mistake: The number of years that Sal has been dead and the number of years that he and Sophia were married, constantly changes. In Season 3 Episode 5, Sophia says that she and Sal were married for 52 years. In other episodes, she says he's been dead anywhere from 22-30 years. If that's the case, given that Sophia is 80 at the start of the show, Sal would have died when she was about 50. Meaning that they would have had to be married since birth to be married over 50 years.