
Tim Emrick |

It hasn't happened to me in Pathfinder yet (I've only been playing it about 4 years, and mostly as GM), but one of my characters in a Buffy/Angel campaign some years back tended to surprise (and vex) me almost as much as she did the co-GMs. Trick was a real wild child, impulsive and a bit capricious, but she's one of the characters I most miss playing.

Kileanna |

Constantly. All my characters end doing the most unexpected things. And I love it.
With my Witch, which I had planned to be neutral, torn between good and evil, she ended feeling well when she made the healing role, refusing to use spells that were pretty good and she knew just because she thought they caused unnecessary pain, and I ended having to shift her alignment to NG.
She was also meant to be some sort of unwilling seductress and she ended falling in love almost at the beginning of the campaign (to the most inadequate person) and spending the whole campaign lovesick as a teen (she actually was a teen) xD

Rub-Eta |
My current character keeps surprising me. His original concept is very outdated: At first I imagined his bloodrage to be some sort of mental trance that sharpened his focus, a very controlled character... Now, almost a year later, he has ended up a nerve wreak, loosing his s+*~ almost once a session where the DM needs to intervene with spontaneous black-outs and loss of will before he actively kills someone in the party.
He has also attracted vampirism (using the Corruption rules with the Fangs manifestation)... so he needs to drink quite a lot of blood on a daily basis - to fix this, he bought four... um... "dancers"... to follow the party along.