DH does this all the time. We see a male health professional - he's a "doctor" (including the awesome male midwife who delivered our ds). We see a female health professional - she's a "nurse", regardless of whether she's actually a doctor, dentist etc.
He's not sexist in any other way whatsoever, in fact is one of the least sexist men I know, but this really gets my goat. Am I being ridiculous or do I need to start some sort of Pavlovian shock therapy to get him to call people by their correct professional title so my blood pressure doesn't go through the roof?!
Incidentally he never actually does this to their face, only when talking about experiences afterwards to me/friends/family. I'm sure a female doctor (or any above examples) would put him bang to rights if he did 