First of all, I love your Titanic avy! I may have to copy you and put one of mine up too!
As for your question... yeah, I'm sure that's happened to me before. Sometimes I'm surprised at the dramatic turns my scenes take; characters start arguing when I didn't plan for them to argue and that sort of thing. Other times I expect a scene to be dramatic, and then the tension never does boil over. Maybe it does have something to do with the mood I'm in when I write; I'm not sure.
What used to happen to me - hopefully not so much anymore - is that I would influence my characters to do uncharacterist ic things for the sake of the storyline I wanted to happen, even if it didn't make sense anymore. Like... certain characters might break up with their boyfriends in completely out-of-character ways because I love a certain song, say Siberia, and don't like songfics, so I incorporate it into my current epic even if doesn't fit, thus ruining the integrity of the character I've spent two years developing. I learned my lesson and try not to let that sort of thing happen anymore. It can be hard to separate yourself from your characters, though.