1. Ordering 'a beer.' No specifics, no details, just general beer.
2. Reaching over the bar to satisfy their curiosity about exactly what's back there.
3. Um-ing and ah-ing and then asking for a 'surprise.'
4. Waiting until right after last call and then asking for a drink.
5. Asking for a drink to be made 'strong,' especially if it involves the concept of doing it as some sort of favour.
6. Asking what beers are on while standing in front of the taps.
7. Complaining about wait times, despite the bar clearly being incredibly busy.
8. Not making room at the bar.
9. Holding money and thrusting it in the general direction of the bartender as an attempt to get served.
10. Elbowing through a crowd and leaning on the bar to get served ahead of people there before.
11. Standing behind many other people at the bar and yet WAVING for some reason.
12. Snapping their fingers to attract attention.
13. Ordering a single drink and then trying to pay by card.
14. Ordering a drink and then sending it back because it's not quite right.
15. Interrupting the bartender while they're talking to another customer.
16. Actually, just ordering single drinks when you're in a group.
17. Complain that a drink isn't strong enough, or that they can't 'taste the alcohol.'
That's because it's been made correctly.