In the example I gave, believing he is Napoleon is the patient's delusion (yep, it's a stereotype). A psychiatrist treating such a patient is likely to regard comments about "my wife Josephine" as being part of his delusion - however, in this case, he is really married to a woman named Josephine (whether his choice of spouse is related to his delusion is another matter).
One case I've heard about 3rd hand dealt with a patient whose condition was improving, and told the doctor that he now realized that what he had been doing was "biting off his nose to spite his face". The doctor, however, was not familiar with English metaphores, and documented the statement as "threatening self-mutilation".
I hope somebody got that straightened out!
I've got a certificate to teach ESL and it's amazing how many metaphores, slang terms and so on -- which native English speakers use without thinking twice about them -- can leave ESL folks going