Casey did not count on her "best friend" Amy spilling the beans on Casey's whereabouts to Casey's MOM, then leading Mom Cindy to Tony's where Casey was. I really liked that little scenario.
Casey did not count on her deeds sending her dad to a psychiatric hospital, on the verge of suicide.
Casey did not count on anyone finding Caylee's body (it's already been said), as she probably thought wild animals would take care of that for her. She forgot about bones, plastic and other things that didn't disappear. (I don't think she forgot. I think that Casey is missing part of her brain function that calls for common sense.)
Casey did not count on people with normal critical thinking skills, people following her trails of receipts, phone calls, shopping sprees on camera.
Casey did not count on Caylee always being a babbling baby who couldn't tell on her. Casey didn't count on losing some of her attention to Caylee as Caylee became part of the family and was doted upon.