Incorrect pricing

I was in Leandra Checkout today at 14h34 and saw Stoney ginger beer 2l at 20.99,and when I got to the till it was 23.99. I explained to the cashier that I saw it at 20.99 and she told me sorry, I went to the manager and told her the same thing she also said sorry, that someone is not doing their. This is misleading as the price was there when I bought it.