Ralph Waldo Emerson famously wrote "A foolish consistency is the hobgoblin of little minds". Though it is apparently left to the reader to decide what counts as foolish or not foolish. If you web search something along the lines of "pictures to drive OCD crazy" you'll find some pages like this one with various photographs of things that are not quite right.
This flashback tells the story of something that the characters have alluded to in the comic at various times. I’ve always called this kind of thing Implied Past. Colloquially they are called Noodle Incidents. Implied past allows characters to seem like they’ve lived complete lives without having to stop and explain every exploit in the middle of the current story. I don’t typically write backstory off handedly. If a character mentions something I’ve worked out enough of the story so that if it ever became important said character could relate the tale. Between Failures