Here are a few more, trying to stay away from wars:
Plague of Justinian: The Byzantine Emperor Justinian I did such a poor job managing a pandemic that they named it after him. This quote sums it up:
Justinian showed no mercy towards the ruined freeholders. Even then, he did not refrain from demanding the annual tax, not only the amount at which he assessed each individual, but also the amount for which his deceased neighbors were liable
https://en.wikipedia.org/wiki/Plague_of_Justinian
1912 Republican National Convention: Taft and Roosevelt could not put their differences aside leading to a walkout from Roosevelt who formed the Progressives. The resulting split vote in the election allowed Democrat Woodrow Wilson to become President with only 42% of the vote. Domesiticaly this halted an American drift towards progressivism and improved race relations. Internationaly Wilson's post war world view and 14 points failed miserably.
https://en.wikipedia.org/wiki/1912_R...nal_Convention
Leaded Gasoline: What seemed like an easy solution to prevent engine knocking issues at the time ended up spreading a toxic substance in high concnetrations around the globe. There was so much lead in the atmosphere during the mid 20th century that future achaeologist will be able to identify skeletons from this time by measuring lead in their bones.
https://en.wikipedia.org/wiki/Tetraethyllead
Partition of India: A short sighted action which caused death and chaos at the time and still has major impacts today. The original plan was to take 16 months for a transition but it was done in 6.
https://en.wikipedia.org/wiki/Partition_of_India