Just had this pop into mind....
When John Wilkes Booth went to assassinate Lincoln, he carried a small single-shot derringer and a knife. He succeeded because Lincoln's usual bodyguard (who carried a pair of revolvers and a bowie) was away, and his replacement left his post to go drinking.
If the usual bodyguard had been there, Booth would have been blown away on the staircase. If the replacement had stayed, it might have been the same, or at the very least Booth would have had to use his derringer and then face a much larger Lincoln, and a Union officer, with nothing but a knife.
Booth didn't have a chance to wait and see if Lincoln was vulnerable that night, and try again later if not. The other assassins had already departed to seek their targets, so Booth either succeeded that night or failed.
Question: how could he have expected his plan to work out? It has to presuppose that there is nobody guarding Lincoln, when there should have been someone there. Did Booth not know that Lincoln would have had a guard, go in without any study or recon, and kill Lincoln by pure happenstance? Did anyone ever give the replacement guard a good grilling?
UPDATE: I have somewhere a photo book on Lincoln...had it for 30 years or so, so it's not recent. In it is a photo of his 2nd Inaugural speech. The photo had been overlooked because in developing the plate someone managed to put a thumbprint right over Lincoln's image, making it seem unusable.
Then someone noticed there is someone looking very like Booth, standing to Lincoln's side and rear.
And right below Lincoln's stand/podium are 3-4 of the conspirators, in a group.
UPDATE: A commenter found a pic online. I'm not sure it's the same pic I saw, but was taken at about the same time. The conspirators have their back to the wall directly below Lincoln's podium. The guy with the wide-brimmed hat, and I think the guy with the military kepi, were two of them, as I recall. Booth was right around that big lantern to the right of Lincoln.