One is not two!

Sometimes when I am standing in a queue in a cafe or a bar, and there is a non-native English speaker ordering in front of me, I often hear things like:
'One coffee please' or 'One pint of beer and one packet of crisps please'.
This is not correct! It should of course be:
'A coffee please' or 'a pint of beer and a packet of crisps please'.
The problem (I think) comes from the fact that in many languages the word for 'one' and the word for 'a' are very similar and hence the confusion. It is an understandable mistake.
So when do we say 'one' in English in this context?
Quite simply, one is not two! I will explain:
Imagine I am standing at the bar with my friend and I order a drink for the two of us. I would say:
'Two pints of beer please'.
The barman pours us the drinks and half an hour later my friend would order:
'Two pints of beer please'.
And again we drink the drinks. We order another:
'Two pints of beer please'.
The barman serves the beers and after them my friend suggests another.
'I can't', I say to my friend, 'but let me buy you one'. I then catch the barman's eye.
'Excuse me... Hello. Can I have....'
Now what do you think I will say at this moment? What is the barman expecting me to say? He is expecting me to say 'two beers please' because that is what I have been ordering all night. But I want to make it clear it is NOT two so I say:
'One beer please'.
And that is the only context where we use one in this way.
Remember, ONE is NOT TWO!