Oh, you mean harmonica....

When my husband took up the harmonica, it drove me nuts when he referred to it as a "harp," since to me that's a large triangular instrument with lots of strings. Fortunately he switched to the mandolin, so the problem evaporated.

In a blues/jazz setting calling a harmonica a harp is standard and fairly unambiguous. But in a folk/trad setting it can lead to confusion.