My character Florence (the pink-haired girl in my icon) started in the story as nervous and clingy. After a few years, she became a puckish, hyperactive tomboy. I never planned on it at all.
My character Opus one day decided to sport a pompadour hairstyle. I wasn't sure why, but he ended up looking great!
I had planned to make my character Mrs. Jovian so sweet and nice it drove the rest of the cast insane. She gradually evolved to be a bit two-faced, stuck-up, and out of touch, and she's actually much funnier and more realistic that way.
Another character of mine, Viktor Petrov, suddenly became diagnosed with OCD at one point. He also lost weight and changed the type of shoes he prefers around the same time. Never planned any of this.
I love it what they do that.