I really love the feeling when you've been stuck on a scene, and then you realize a way to make it work that genuinely excites you. Like "Oh, I've really needed these two characters to meet, but haven't figured out a good reason, but I just realized they both get injured around the same part! If I move the timeline around a little, they could meet in the hospital, which means they'd meet when A just found out she'd been betrayed!"
A lot of the time, it's realizing something I assumed doesn't need to be true. Deciding to cut a scene I love or kill off a character I wasn't planning to can be upsetting, but realizing it was the odd shaped piece that was keeping everything else from falling into place is really satisfying.