I think you want to post some possible examples...
...But here are a few possibilities:
A time traveler decides to kill baby Hitler, to prevent the Holocaust and World War II. Instead of killing him, he just wings the kid, sparking a lifelong rage and resentment that ensures the events of the 1930s and 40s.
Sparrows ruin a harvest in China. Chairman Mao directs that all sparrows should be killed. As a result, a generation of locusts, uneaten by the missing sparrows, eats even more crops, causing a famine that kills millions of people.
In situational irony, you set out--with the best of intentions--to prevent or redress a terrible wrong, but instead of preventing it, your actions actually cause it to happen or to be a lot worse than if you had just stayed out of it.