A few months ago I was buying breakfast at a McDonald's at O'Hare Airport. A Chinese man, who spoke no English at all, was trying to buy a hamburg. Seriously, he couldn't even say "hamburg", he said "han bao" in Chinese! The cashier seemed to be quite experienced with international travelers so he just brought out a menu with pictures. Of course, this man didn't know there were so many different "han bao" in the US. So being the good Samaritan, I tried to help, talked to the cashier to help him make the order while talking to him in Mandarin.
This man didn't look at me at all the entire time, nor did he talk to me directly, let alone saying "thanks". He's got his food and went back to his table, where a couple of same mid-age Chinese men were sitting, without food.
I don't know if he felt "losing face" because a young Chinese woman helped him, or he was used to have others' service, or he was just not used to express appreciations like many Chinese, whatever reasons, I felt really stupid for helping this stranger. Well, it turned out that I didn't stop helping wandering Chinese tourists later, but I was deeply saddened by this incident.