The way I heard it (this absolutely reeks of urban legend), but there is indeed at least one spook in the Florida version of the mansion that the imagineers didn't count on. According to the castmembers I've spoken to, sometime in the early '80s, a little boy died somehow, and the family opted to have him cremated. Knowing how much her son loved the mansion, she respectfully requested to have the ashes sprinkled inside. The powers that be naturally were disinclined to acquise to her request (I guess they figured it'd have been a
PR disaster if word got out about it, and it would have). Undaunted, the mother hopped on the ride, and stealthily spread the ashes herself.
There were a few castmembers several years back who claimed that on certain nights after the rush was over, it was possible to glimpse a sad, lonely looking boy riding in the space between the doom buggy loading and unloading areas. From what I've heard, the poor little guy always appeared to be pretty unhappy. I suppose that makes sense--I know I wouldn't have wanted to have ridden the Haunted Mansion alone at his age.
Again, I'm not sure how much veracity there is to this tale. It could have just been a story the castmembers of the 1997-1999 era liked to pass on to curious geeks like myself.
Not sure if this has any connection to the one about the young boy, but Athena (the castmember I became the friendliest with) confided that during her tenure, some of the guys were goofing around with a camera in the buggy unloading zone one night after hours, and when the photos were developed, there was something white and vaguely humaniod in among the crew. No one could offer a good explanation for it. I honestly wish that I could have seen the shot for myself.
Happy hunting!