Problem definitely not limited to mandolins. I play Autoharp, and am amazed how many people identify it as a "harpsichord." I figure: three-syallable word, has "harp" in it, I'm playing chords, there you go... On the other hand, a mandolin has the general outline of a guitar or ukulele -- sound box, neck, strings, etc. -- while an autoharp looks like a zither (it is a kind of zither), and a harpsichord looks like a piano. So visually there's less excuse.
And don't even get started on hammered dulcimer! I play with several dulcimerists, and they seem to spend as much time answering questions as they do playing the thing. I saw a Boston street musician who had a sign attached to his dulcimer saying "1. It's a hammered dulcimer. 2. I can't talk to you about it when I'm playing." Maybe someone should sell a smaller version of the sign for mandolin players.
Gibsn: '54 F5 3pt F2 A-N Custm K1 m'cello
Natl Triolian Dobro mando
Victoria b-back Merrill alumnm b-back
Stradolin Vega banjolin
Sobell'dola Washburn b-back'dola
Eastmn: 615'dola 805 m'cello
Flatiron 3K OM