WARNING: The Following Article Contains Spoilers For The "Nora" Episode of The Flash.
The season 5 premiere of The Flash subtly covered up a major plot-hole, which was introduced into the series during its fourth season, when Ralph Dibny - The World Famous Elongated Man - was introduced into The Arrowverse. The Flash is no stranger to complicated plots, regularly incorporating multiple universes and time travel, but instead of creating more plot holes, season 5 is patching them up.
The problem goes back to the first season of The Flash and the episode 'Power Outage'. In this episode, an electricity-draining metahuman called Blackout targeted STAR Labs and Dr. Harrison Wells, whom he blamed (not unfairly) for the particle accelerator explosion that ruined his life. When Blackout confronted Dr. Wells and accused him of not caring about all of the people his experiment hurt, Wells quietly replied that he cared very deeply and he had memorized the names of every person who died or went missing that night, so he would always remember the price of his failure. The names Dr. Wells listed off included a number of superhero secret identities from the comics, including Ralph Dibny.
This didn't mean much until the fourth season of The Flash, which introduced Ralph Dibny into the show as a recurring character. The disgraced cop turned private investigator was one of the 12 "bus metas" created by the super-villain called The Thinker, to give him the powers necessary to enact his sinister plans. While the vast majority of fans didn't remember Dr. Wells' speech from the first season, there were some who wondered just how Ralph could be around when Dr. Wells remembered him as having died.
The fifth season opener 'Nora' provided an artful solution. Confronted with the existence of Nora West-Allen - Barry Allen and Iris's West's daughter from thirty years in the future - a confused Ralph asked how time-travel was possible. It turned out that, in all the excitement involving The Thinker and Ralph's imminent death over the past year, nobody had ever bothered to explain to Ralph that speedsters can time-travel, that Team Flash was friends with a group of time-travelers and that there are multiple parallel Earths. This ultimately leads Ralph to ask Cisco Ramon and Caitlin Snow one question - "If Barry can go back in time, why didn't he stop The Thinker from killing me?"
As they struggled to think of a simple answer, Ralph replied that he was joking. Though he's a detective rather than a scientist, Ralph figured that Barry didn't try to change the past because it wasn't worth the risk of making things worse by trying to fix things. Ralph then joked that if Barry had gone back and changed the past trying to save Ralph's life, he might have created an alternate timeline where Ralph died in the particle accelerator explosion instead of being killed by The Thinker.
As fans of The Flash will remember, Barry did once try to go back in time to save his mother's life in the season 3 premiere "Flashpoint," which was based on the classic comic book storyline of the same name. Given that Barry's actions there had a number of unintended side-effects, such as activating Caitlin's meta-gene, changing John Diggle's daughter Sara into his son John Jr. and killing Cisco's brother, suggesting that Ralph's existence is another unintended side-effect of Barry's altering the timeline of The Arrowverse is a brilliant conceit. We'll just have to hope the show's writers don't stretch their luck trying to use this to excuse more plot holes in the future... or the past.