I mean, Columbus, right?
Hard to pick just one? Probably also depends on how you are going to define it?
But things like the Hungarian uprising; where there was encouragement to rise up against the harsh communist rule, only for the Hungarian freedom fighters to then receive no backup or support at all from the west. Similarly the Americans encouragement and enabling of similar action in Cuba.
But there are many things you could point; people like George Soros shorting the pound to make himself a cool billion, to the detriment of millions of ordinary people.
Many smaller examples though. Lord Ashcroft the tory donor didn’t get a cushy job he had wanted when David Cameron became PM; so Ashcroft spread the “piggate” story (which is completely baseless). But now David Cameron will always be the guy that may have put his dick in a dead pigs mouth… when you throw shit; even when that shit is bullshit, some will stick. No fan of Cameron; but Ashcroft equally can go fuck himself, the tax-dodging twat.
Colonization, effects will be felt now and forever
Genocide of the Native people that helped you survive and continued violation of treaties to this day
Porn