Another one is they assume you play the music of the only popular harp players they can think of. One of my coworkers introduced me to a woman we sometimes work with and told her I was a great harmonica player. She said, "Oh, I bet you play a lot of Neil Young!" I told her I've never played any Neil Young that I can think of and that I'm mainly a blues player.
And, slightly related, guitarists/band leaders at jams always assume you only play second position. I don't know how many times someone has told me something like "This song is in G, so get your C harp." No, I don't want to know what second position harp to use, I want to know what key you're in so I can decide how I want to play.
Just a couple of pet peeves to get off my chest this morning.
