Warning: The following contains SPOILERS for Legends of Tomorrow's season 7 premiere.
Some have complained that the superheroes making up the Legends of Tomorrow seem to forget they have superpowers half the time, but Steel just altered the timeline by mistake by using his powers at the worst possible moment. This was the end result of a number of bad choices and dumb luck, which is par for the course for the Legends, whose unofficial motto is "Sometimes we screw things up for the better." Ironically, this motto was coined by Nate "Steel" Heywood himself, who is also probably the most guilty of all the Arrowverse heroes when it comes to forgetting their powers when it would be helpful.
The plotline of Legends of Tomorrow season 7 finds the titular team of time travelers trapped in Texas in 1925, following the destruction of their time-ship, the Waverider. With the wreckage being probed by locals, and the black box containing vital emergency supplies confiscated by the local sheriff, the Legends had to work quickly to avert a paradox. Nate Heywood hit upon the idea of posing as Federal Bureau of Investigation director J. Edgar Hoover, knowing a sheriff of that time would immediately comply with any order given by the legendary lawman. Unfortunately, the real J. Edgar Hoover happened to be in the area chasing a notorious gang of bank robbers and the ruse quickly fell apart.
This led to some of the Legends posing as a new gang of bank robbers and a dramatic car chase, as they tried to lure J. Edgar Hoover away from the Waverider crash site. While they were successful in this regard, the Legends still couldn't escape the tenacious Hoover, who survived being thrown off of a speeding car and tried to shoot Nate Heywood at point-blank range. Luckily, Nate reflexively activated his power to transform his body into living steel in time to save himself. Unluckily, the bullet ricocheted into J. Edgar Hoover's forehead, killing him instantly, nearly 50 years before he was destined to die of a heart attack. This is one of the reasons why the Legends are considered the least capable of the Arrowverse's many superhero teams.
It's highly ironic that it was Nate Heywood using his superpowers that brought about the Legends latest screw-up. As a professor of history, Heywood usually uses his knowledge to direct his team in averting paradoxes and changes to the timeline that might damage humanity's destiny. Nate is also probably the least inclined of the Legends to use his powers, being a thinker rather than a fighter, which is doubly ironic as he's currently the only member of the team (besides co-captain Sara Lance) to utilize a superhero name or possess a superhero costume.
Nate Heywood rarely wears his costume nowadays and it has been several seasons since he actively presented himself as Steel in public. Despite this, he's reportedly still lobbying for a place in the Arrowverse Justice League. Unfortunately, given his accidental actions in the Legends of Tomorrow season 7 premiere, the chances are increasingly slim that he'll ever be offered a seat with his symbol on it at the big round table.