Yes, I have seen similar things on other cruise lines, like one time I had a coupon for two for one wine, and I showed it to the bar tender and he agreed it was good. He poured the wine and brought the slip, but charged me for both glasses. Unfortunately I did not notice he had done that until later in the cruise. I suppose i could have protested but it didn't really sink in until later.
Basically - you have to watch what you are signing, because you never know. A lot of people just take it for granted that the ticket is always right.
For that matter, I recall a restaurant here in Phoenix where i got an incorrect ticket "by mistake" every time I went there. By the fourth time there I said to my companion "just watch, I'll bet you they make a 'mistake' on the ticket" and sure enough they had "mistakenly" given me a ticket for another table who had charged up more.
I think far too many people just sign whatever is handed to them and do not check the ticket.
Things do happen - and I do think in the bar business two for one generally does mean two glasses for the price of one. But she should not have had an attitude when you said you only meant to have a total of two glasses, not four.