On some occasions when I have played: (not at the same time!) an Autoharp; a Zither; a Harmonium; a small antique pump pipe organ----
   "Is that a Harpsichord?"

Many years ago, I was playing piano accompanyment for a Bassoon player at a recital. Two elderly ladies were trying to figure out what the solo instrument was, and one of them answered loudly enough for the first few rows (& us) to hear, "Myrtle, don't be ridiculous! That's a Sousaphone--you know, just like Tommy Dorsey's brother, Jimmy, used to play."

But the best one I can remember was when I was playing accompanyment on an antique keyboard with a Renaissance instrumental group for a large Rennaissance festival audience, and the MC, Dean of a local Univ Music school, correctly read from the program, " . . .and next we have the Renaissance ensemble with Kaleea accompanying on the Vaginal."
          instead of the Virginal