One that springs to mind for me, was Operation Mincemeat in WWII. Such a risky, crazy mission to float the dead body of a vagrant named Glyndwr Michael onto Spanish shores disguised as a high ranking British officer named Captain (Acting Major) William Martin in the hope that the body finds its way to the shore intact, carrying 'British secret plans for Invasion of Sardinia and Greece'. And is somehow found by a high ranking German officer…
So much could have gone wrong. The body was already beginning to decay when thrown into the water.
The tides were uncertain.
The autopsy would almost certainly have revealed the corpse being older than was needed to provide the bluff, thankfully an insider at the autopsy tried to hide this by saying something along the lines of, "let's just hurry this and go get lunch, it's too hot". So the autopsy was ruled as drowning, which furthered the bluff (rather than eating rat poison, which is how Glyndwr Michael actually died).
On top of all of this was some serious back and forth bluffs and double bluffs via spies, deliberately crackable and breakable messages.
Major Martin's 'death at sea via planecrash' was even reported by The Times, and coincidentally his death was placed amidst two other (real) Officers who had died at sea, further enhancing the bluff.
Somehow against all odds, it worked. Hitler was convinced and changed his tactics. Shuffled and rotated his military hardware and troops accordingly.
On an amusing note, I remember fondly reading about something that happened in Africa(?) not too long after the deception had been revealed as a bluff and D-Day occurred.
I think I remember reading about some British Officer somehow accidentally leaving some extremely important documents, which were then obtained by a high ranking German Officer. However this time, those offensive plans were considered bluffs and thus the Germans refused to be 'fooled' twice, and ignored the documents as nothing more than lies.
Almost a reverse Operation Mincemeat.
What are some other examples of how crucial historical events could have gone so wrong at any point and yet somehow worked?
Source: reddit post