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Thanks for your answers so far, Squad I like yours!
Boo, in this store, the queue goes left to right down the serving counter, so customers tend to stand side by side as they are being served, I guess in this case the two customers were stood quite close to each other which is why the mistake was made.
Yes I agree that some people just 'wait' to be offended so they can make a complaint!
In this store, we have a lot of students coming in and office staff. They tend to come in as a group, so quite often staff are serving more than one person at the same time, I think this is why the mistake happened.
A shame though that this person is so sensitive to the way others perceive their ethnic culture that they are so easily offended by something of this nature.