Abraham's flashback sequences are the very first scenes in the show's history to take place outside of Georgia. In the comics, Rick hailed from Cynthiana, Kentucky, but his hometown was altered to King County, Georgia for the show. Abraham's flashbacks take place in Houston, Texas.
Abraham's wife in the comic books is Beth not Ellen.
When Maggie (Lauren Cohan) talks to Eugene (Josh McDermitt) about his haircut by the decommissioned firetruck, he is reading H.G. Wells' "The Shape of Things to Come.' The plot of the book is about a doctor who had visions of an established future world state caused by humanity's problems, such as wars and other disasters.
This episode was almost banned by the censors.
After the second flash back from Abraham's past when you first see Ellen's face and the children, the group is leaving the bookstore heading towards the fire hall and you see them walking through a puddle with rotting cardboard and a dirty hairband that is the same as the one that Ellen was wearing.