Once my older brother took a taxi in Madrid. He often goes to the capital for business. During the trip he was called to his mobile phone. When he heard him talking, the taxi driver stopped and ordered him to get out of the cab. He had recognized that he was speaking Catalan.
My best friend is from Menorca. Her native language is Catalan. We both went to uni in Barcelona. The Catalan variety of Menorca is quite different from Barcelona’s, which is considered “the standard”. She had to give a presentation for a pragmatics course. After she gave the presentation, one of the FOUNDERS of the linguistics department (first linguistics department in Spain) told her: “you probably did very good, but I didn’t understand much”. I (and the rest of the students) understood her perfectly fine.