In My Mother the Carjacker (2003), Homer stated that Mona missed Bart's birth. But here, he reveals that Mona visited them in the hospital when Bart was born and thus she knew she had a grandson already.
16-year-old Homer receives a postcard from his mother meaning he knew his mother was alive at the time. This contradicts the events of Mother Simpson (1995), when Homer just finds out she's alive and didn't know about the packages she sent him every week because he refused to tip the letter carrier. If he found out she was alive in his teen years, he wouldn't have thought she was dead in his 30s.