The first was getting the Halberd, which was all luck dependent. I got into Darkroot Basin, found the Black Knight, then started to backstab him. For some reason, I think his animations got really... stiff. Like, he could still move and attack, but it looked like he was walking slower and his swings were slower. When I did backstab him, he didn't respond much, other than hitting the ground at a weirder angle than usual. I was pretty scared, honestly. Fortunately, I killed him and got the Halberd in the process, so I didn't need to restart my file.
The second was during the Iron Golem fight in Sen's Fortress. I had Tarkus and another phantom assisting me. We had staggered it, then it fell... and fell through the floor, along with the human phantom. You know how normally you can make the golem off of the roof if you angle it? It wasn't near the edge at all, and yet it fell straight through the floor and it counted as the falling off kill, so I beat it, regardless. Seeing that made me say, "What." over and over for a good 10 seconds.
It was an interesting experience, to say the least.