I get a similar experience with regular undercharging in my local pub. Server takes order, wanders past cashier telling her what I've ordered, then server goes and fetches my order. I used to point out the missing beers at the end of my visit, never got any positive reaction, only a new correct bill. Not a problem to me but I've given up telling them as it's their system that fails every time. Some of the servers though have now taken to asking me when I get my bill at the end if it is correct or not, I suspect I have gained a negative reputation by trying to help them out.