Or do you mean that people who say, for example, that ghosts exist are saying this because they are illegitimately extrapolating a theory that works in one domain into another?
Yes.
When, in ordinary circumstances, you encounter a debate between two people about whether ghosts exist, do you think one of them is right and the other is wrong?
Usually yes, since people rarely argue whether ghosts exist in mythology. But a discussion about whether numbers exist is almost always a confusion about domains, since numbers exist in the mind, just like ghosts.
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.