In the first flashback of Marge telling Homer she's pregnant (with Bart), they are in their present house on Evergreen Terrace. They did not move to this house until after Bart was born.
In this episode, Marge reveals how Homer reacted to learning of each of her pregnancies - with him ripping out his hair and running up the stairs in their current home (purchased when Bart was young and after Homer knew Lisa was coming.) In a previous episode, however, it's shown that Homer found out about Bart at Dr. Hibbert's office, and reacted with "d'oh!" Marge informed him of her pregnancy with Lisa in their old apartment and he reacted positively.
When Marge tells Bart and Lisa that she is pregnant, she is visibly showing that she is with child. In the next scene in the bedroom when she tries to tell Homer, she is no longer showing.
At Marge's baby shower, Ruth Powers is shown among others, even though she hasn't moved to Springfield yet.
When Homer carries Marge into their bedroom after their night out celebrating, the doorway is suddenly located where the en-suite has always been, and the hall can be seen outside the door continuing on past the side wall, when this wall is the end of the house. Later in the episode, the en-suite is back in this location and the bedroom door (and hall) are at the front of the room where they have always been.
In one scene, acid rain starts to fall and it dissolves Homer's jacket. Not only does acid rain not act like that, but rain that caustic doesn't even seem to affect the exposed skin of Homer or the security guard.
Homer is telling the story of Maggie's birth. In the flashback scenes, there is a picture of Maggie on the wall of the Simpsons' home even before she is born.
Both times Homer rips his hair out and runs up the stairs screaming over Marge's pregnancies, a framed picture of Lisa in her Sunday clothes is visible near the top of the stairs.
During the flashback, their neighbor Ruth is shown at Marges baby shower for Maggie. Ruth had not yet moved into the neighborhood.