I attended a wedding yesterday and was told that there would be 1 keg and once that ran out you would have to pay for anything else in addition if you wanted anything besides that you would have to pay for it as well as there was a bar attached to it. Well, I'm not one to ever carry cash on me as I live in a big city and this back near my hometown. I get to the wedding reception and find out that even the keg is cash, everything is cash and guess what, I don't have cash on me and do they accept debit..definitely not. So I have to leave the reception before it starts to go to my bank 5 minutes away and come back.
So please, don't do a cash bar without proper planning its tacky if your guests don't expect it. Granted, I didn't mind the cash bar once I was prepared as I'd rather have the option to buy a drink vs not drink at all since I recently went to a dry wedding but I would of liked to have some head's up to have some cash prior to.