Sideshow Mel is shown a few times to be Krusty's sideshow in the flashback, whilst Sideshow Bob was supposed to be Krusty's sideshow at the time, before the events of Krusty Gets Busted (1990) when Bart and Lisa were 8 and 10 years old, respectively, while in this flashback from 6 years ago they'd be 4 and 2.
There is a Dr. Marvin Monroe Memorial Booth, despite Marvin Monroe not thought to be dead at the time.
Moe informs Homer there's a space over the garage that the family never noticed exists, and Homer starts staying there. Then while in there Homer finds mouse traps with cheese which he tries to steal. If they didn't know about this space, they couldn't have placed mouse traps, never mind loading them with fresh cheese for Homer to steal.
Smithers has yellow skin rather than black skin.