Despite hailing from either side of the great comic-book divide between Marvel and DC, The Flash and Iron Man have the exact same character flaw that lands themselves, and their teammates, in hot water all too often. Often proving incredibly dangerous, if not fatal, both Barry Allen (Grant Gustin) and Tony Stark (Robert Downey Jr.) show a determined resolve to not learn from their past mistakes. There is plenty of evidence for this across Iron Man's appearances in the Marvel Cinematic Universe, as well as The Flash's seven-season stint as part of the Arrowverse on The CW.
First appearing on the small screen all the way back in 2014 in The Flash, Barry Allen and his team have become cornerstones of the ever-expanding superhero universe known as the Arrowverse on The CW. Now well into its seventh season, Barry shows no signs of learning from his previous mistakes, having created some terrible timelines; a trait which he, unfortunately, shares with one of the MCU's most famous faces: Iron Man. Tony Stark has undoubtedly earned himself a reputation as one of the most hot-headed and reckless members of the Avengers over the years, and this is certainly reflected in his decisions to seemingly ignore past mistakes in favor of doing things his own way.
Perhaps most notably, Barry seems to willfully ignore all previous repercussions of his attempts to mess with the Speed Force. Despite being an incredibly powerful force of nature that Barry cannot control, he seems determined every time that he can bend it to his will and deliver the result everyone wants. Yet, messing with the Speed Force results in disastrous circumstances that are only avoided thanks to Barry's team of intelligent friends. This is mirrored by Tony Stark's journey throughout the MCU, paying particular attention to the birth of Iron Man's most important villain, Ultron (James Spader) in Avengers: Age of Ultron. As heroic as these characters may be, they are also often the architects of their own team's problems.
Throughout the Arrowverse's existence, the Speed Force has allowed the Scarlet Speedster and others to time travel, and bend the rules of the universe. As a living entity, the Speed Force has recognized this abuse and has sent Time Wraiths to stop those who would seek to manipulate the Speed Force for their own gain. Barry has fallen victim to being pursued by Time Wraiths, especially during his first foray into the Speed Force. It is thanks to his team that Barry was able to fend off the Time Wraith. Other abuses of the Speed Force have seen Barry lose loved ones time and time again, which only prompts him to further abuse the Speed Force in the hope of reversing his mistakes.
Iron Man's journey in the MCU, particularly during the Avengers sequel, showcases many of the same traits that Barry shows in his disregard for the consequences that befall his team when he interferes with the Speed Force. After retrieving Loki's scepter from Hydra, while seeing his worst fear thanks to Scarlet Witch, Iron Man sets about interfering with the scepter in a bid to do what he believes is right. In disregarding his fellow Avengers, Stark creates the murderous Ultron who nearly brings about the end of the world thanks to his plan to transform Sokovia into a meteor, which could be the origin story for another MCU villain. Yet, the final act of the film features Stark doing almost exactly the same thing in wanting to put JARVIS into Ultron's perfect body. While this may then go on to become Vision, the disregard for his past mistakes is still there, and it causes headaches for the team as a whole.
Both The Flash and Iron Man have this fatal character flaw, that has cost them dearly in the time that they have been onscreen. Unfortunately for those around them, there has been little in the way of evidence to suggest that the former will ever change, and the latter certainly did not, though it had no bearing on the heroic and sacrificial ending that Iron Man found in Avengers: Endgame.