On more than a few occasions now, I have
1. gone to a bar
2. given a credit card to run a tab
3. drank quite a bit
4. been significantly undercharged
Like charged for 2 $8 beers when I drank at least 5. So how much should i tip on the $16 bill? $24 to cover what I should have been charged? $8 to tip 50%? How do I not know this answer at this age.
Why does this not happen to me? I used to have to dispute the charges because the bartender would put his friends drinks on my card. Luckily I would only have 2 or 3 drinks, but when the bill came at 7 drinks, I knew it was wrong.
To your question. The proper thing to do would be to tell them you had two additional beers. But, who does that, right? Especially because that beer costs the establishment $0.25 per beer to purchase, and they charged you 32 times the cost.
At the very least, (if you don't correct them on what they should have charged you) tip on $40, which should have been the bill total. Please don't screw over the server, that tip is what they count on.