Not exactly a misunderstanding but... my dad (a professor here in the U.S.) had a close friend and colleague, a Spaniard, who would go off to an intensive language summer school thing every year to teach American college students whatever esoteric Spanish literature was his specialty and only spoke Spanish the entire time.
Whenever he got back, he would spontaneously start talking to us in Spanish, suddenly realized we didn't speak Spanish, then restart again in English. It didn't embarrass him or anything, but it amused me when he did it.
Oh man this happens to my mom all the time, in both languages too. She'll speak English to people in mexico and Spanish to people in Canada. Cracks me up every time, but sucks when we're trying to pass as locals in mexico.
I had a similar experience when I was learning English where I was trying to give something to my friend, eventually I realized I was just repeating a number (10) at her. Ten means "take this" in spanish.