A very divisive concept in the recent annals of DC history has been that of Flashpoint. The comic event depicted Barry Allen going back in time to save his mother from death, hoping to give her a better fate. Little did he know, his actions had a ripple effect on the entire DC timeline, and the dire consequence of that forced him to reset it.
Source:https://www.cbr.com/harley-quinn-flashpoint-dark-reinterpretation/