Famously mismanaged DC crossover from the early 1990s.
Waverider is sent back in time from a dystopian future to prevent a tyrant from gaining power. Armed only with the knowledge that Monarch used to be a superhero, Waverider touches each superhero with his or her own Annual for a glimpse of the future.
The problems arose when word leaked early in the summer that Monarch was going to be Captain Atom. In a ridiculous attempt at damage control, the editors at DC decided to re-write their ending on the fly. At the last minute, Captain Atom was allowed to continue as a hero while Monarch appeared and brutally murdered Dove from also-ran Titan wannabes Hawk & Dove. When Dove is killed, Hawk goes crazy, murdering Monarch who is revealed to be a future version of Hawk and putting on the Monarch armour himself.
It was a ignominious end for Hawk and Dove and was received badly by fans who sensed the rush job.
Interestingly, Captain Atom has recently been seen donning something similar to the Monarch armour so it's possible that there will be a return to the originally planned ending to this best forgotten series.