Waiters overcharging for more tips?

I attended the metalfeast event hosted there on 27 Feb 2016. The whole day I've been paying R22 a beer (despite the bottles saying recommended selling price is R6, I understood they had to make a profit). Towards the end of the night, when the last band was playing I went to buy a beer for my friend in said band for when he is done. When i got to the bar a waitress asked me what I would like and I gave her my order of 1 beer. Whilst waiting for the barman I took out R22 just for the waitress to inform me that all of a sudden its R30 a beer. I told her but I've been paying R22 the whole day and all she had to say was that the prices increased the week before. I wasn't in the mood to argue so I gave her R30 and took the beer but that is ridiculous. As I was leaving she even had the cheek to click her tongue at me for not tipping her. <br> The reason I'm only reporting this now is because I mentioned it to a friend of mine that was also there that night and he told me that they charged him up to R35 a beer. I wonder how many other people the waiters conned that night and if this happens on a regular basis?