WARNING: The following contains spoilers for Harley Quinn #66, by Sam Humphries and Sam Basri, on sale now.
Harley Quinn has become one of the more meta figures in the DC Universe. She often conveys information that only a character with knowledge of the comic world around her would be aware of. This gives her the chance to target more foundational problems in the universe than other heroes and villains.
But now it seems as if Harley is going for the biggest target she could possibly aim at: the very concept of Crisis crossovers themselves. And it could bring down in the entire multiverse.
Harley spends most of #66 outside of the regular reality. Initially recruited by the Lords of Order and Chaos to serve as a new champion, Harley quickly turns on them in an attempt to bring her mother back to life. However, the forces reveal that it wasn't their machinations that led to her death. The Forces allow Harley to speak with her mother one last time so she can come to terms with her passing.
Harley decides to give up her position of power so her friend Mirand'r can become the champion in her stead. Coming to terms with what happened to her, Harley decides to leave Coney Island and her friends to forge a new life. But peeking out of her bag is a comic, focusing on Harley herself. Wearing her classic costume, Harley and Booster Gold ride the Ferris Wheel, happily getting to know one another.
Harley suddenly looks up, feeling her "crossover notification" going off. The insignia associated with the Totality from Justice League appears in the sky. Furious that her story is being taken over by another crossover, Harley decides that she's had enough of crossovers ruining her fun. She announces her intention to go back in time to the first Crisis event - and end it before it ever began, hopefully preventing the domino effect of any of the other Crisis stories.
Crisis Killer Squad
Harley's meta-sense of the comic world around her gives her the chance to identify the crossover problem at the source. The teaser for the next issue even hints that Harley will put together a team specifically designed to "kill" a crisis. Having access to Booster Gold's time travel abilities is a major first step forward. Seeing the pair together on good terms is particularly sweet because in the events of Heroes in Crisis the pair were pitted against each other.
It's unknown, though, exactly which Crisis Harley plans to target. The most obvious choice would be Crisis on Infinite Earths. The Marv Wolfman/George Perez story radically altered the face of the DC Universe and started a chain of events that is still playing out ]today. It's one of the largest scale stories that DC has ever produced, even compared to the modern-day. However, it technically wasn't the first Crisis event.
Various other smaller stories (that often paired together heroic teams from across the multiverse) have been known as a Crisis. The earliest were "Crisis On Earth-One" and "Crisis On Earth-Two". With DC reembracing the JSA, Harley could end up going all the way back to the first Crisis, which was the first crossover between the Justice League and JSA.