Where I live now, almost all restaurants add a tip, and then leave a line for "additional tip." Usually, if they want to add on a mandatory tip, it would be on the receipt that you sign.

I did once have a server forge a tip, I left a 10.00 tip, and she put another "o" on top of the 0 to the left of the decimal point, making it look like 18.00. The restaurant had the receipt, it was clear that there were two kinds of writing, and the manager was very nice about it. (I assume that the server was told about it, and if this wasn't the first time, was told not to bother coming in again.)

I've also had a bad habit of putting a tip on a receipt, then taking the copy I tipped and signed with me. The charge on my credit card has always been the amount I signed for. (In these cases I've had to either call the restaurant to have them tip the server, or go back in and find the same server. I don't like to stiff people who work hard.)

Credit card agreements do permit late-posted charges, for example, if you sign for your bill at a hotel and then a room service or parking charge comes through that wasn't on your bill at the time you signed it. That's valid.

But without knowing just how much you were charged for the beer itself, or what the receipt said, it's hard to know what your case is here. Why don't you ask the restaurant for a copy of the receipt?