My family hosted a Taiwanese exchange student when I was in high school. One day, he and I were out seeing the town. He discovered a Chinese Tea house in an out-of-the-way corner, and we stopped in.
The owner, a Chinese expat, greeted us enthusiastically and personally sat down to serve us tea. The conversation was lovely; she was explaining the source of the tea, the preparation and serving method, etc. and I was just enjoying the experience when she offhandedly asked where he was from.
When he responded, "Taiwan," she immediately frowned and made some terse comment in Mandarin, to which he retorted, also in Mandarin. There was a tense exchange, and then she abruptly turned to me and started asking me questions again in English, changing the subject.
I had not yet learned of the One-China stance, so when we left I asked him what happened. He told me that she admonished him for not saying he was from "Chinese Taipei", and apparently some other nasty remarks he decided not to translate.
It was such a shock to see that level of nationalism exhibit itself in real-time. She was genuinely offended and angry, not annoyed. I suspect if I hadn't been there, she'd have told him to leave immediately.