Of the number of attempts on Abraham Lincoln's life, the one I found most interesting was the conspiracy of John Wilkes Booth, mainly because of it's timing.
Think about it: on April 14, John Wilkes Booth and his associates set in motion a conspiracy to kill the President, the Secretary of State, and possibly[1] the Vice President. None of them went as planned: Atzerodt, if he was supposed to, failed to even confront Andrew Johnson, Lewis Powell only managed to cut up Seward's face, and John Wilkes Booth ended up getting thrown off the balcony after failing to either shoot or knife the president.
Three days later, William T Sherman meets with Joseph E Johnston at Bennet Place[2]. Over the next three days, they negotiated the surrender of the bulk of the remaining Confederate forces. Johnston insisted that certain political issues , such as the re-establishment of state governments after the war, needed to be resolved; Lincoln, when he was informed, sent his conditions for this surrender via telegraph, in what would become known as the ten-percent solution. On April 20 -- against the express instructions of Jefferson Davis -- Joseph E Johnston surrendered to William T Sherman, effectively ending to US Civil War.
But what if: Seward and Johnson goes as they did historically, but Booth doesn't get noticed by the Unknown Patron[3], so he doesn't waste his one shot on Henry Rathbone. Just to add some drama into the mix, let's say he even escapes for a period after, if instead of losing his knife wrestling with the president, he uses it to slash anybody who gets in the way of his escape route.
So: Andrew Johnson, a drunkard from Tennessee, is President of the United States. The North, I would have to imagine, is furious. A clean surrender at Bennet Place seems pretty unlikely now, especially with Johnston demanding political issues being resolved.
So what happens now?
P.S. I actually do know of one alternate history novel that deals with this, where a Bennet Place surrender gone bad leads to the survival of the Confederacy. I actually don't think much of the idea, and it's kind of presented as a tongue and cheek idea anyway; the whole thing's a set up for saying that the Unknown Patron was really a time traveler [

].
OOC: Hello everybody! This is my first thread, so I hope it goes well. Notes:
[1]Both OTL and TTL, JWB left a note at the VP's residence: ""I don't wish to disturb you. Are you at home?"
[2] How it went down OTL,
http://www.nchistoricsites.org/bennett/main.htm
[3] TTL, historical name for an unidentified member of the audience who cries out "He's trying to kill the president!" at a key moment