Half-Decent Proposal - S13-E10
Corrected entry: Artie says that he has been thinking about Marge for the last 20 years. However, we have seen in previous episodes that Marge got pregnant with Bart just out of high school, and he is 10. They can't have graduated more than 11 years ago.
Correction: In the episode "that 90s show", Lisa discovers that Homer and Marge are nearly 40 and Bart is 10 so they must have had Bart when they were 30. Assuming they graduated high school at 18, that gives 10 years before having a child and the 10 years after which is 20 years, so Artie is correct.
Half-Decent Proposal - S13-E10
Corrected entry: Artie Ziff is upset to discover that Marge is married. But not only does he address her as "Marge Simpson" and not "Marge Bouvier" at first, but 5 series ago he met Homer and Marge at a school reunion and Homer said that Marge was his wife.
Correction: Marge got pregnant with Bart in 1980 (See Episode: I Married Marge, Season 3). They married soon after this. This is to do with the slightly larger plot hole of everyone in the Simpsons always being the same age (at least they stopped aging after 1990 when the show started). This is a cartoon convention and not a mistake.