1. Ask a stranger to lunch "to pick their brain"
If you value someone's opinion, then their expertise is worth more than the cost of their lunch! Ask someone to lunch because you want to meet them, not because you want them to consult with you for free.
2. Extend an invitation to a stranger and specify the restaurant, date or time
You don't want to signal to anyone that your needs supersede their own! If you invite a stranger to lunch, the question is "Would you have time to have lunch with me?" Don't ever, ever specify a restaurant, a date or a time when you invite a stranger to meet you.
3. Invite someone to lunch at a spot near you but far away from them
It is impolite to reach out to a friend and say "Want to have lunch next time you're close to my office?" If you want to see them, you can meet them halfway.
4. Ask a stranger to lunch without offering to pay for their lunch
If someone you don't already know is willing to have lunch with you, you must pay for their lunch.
5. Invite someone to lunch without checking to make sure the restaurant will be open for lunch - and making a reservation if appropriate
Do not subject your lunch partner to the pain of waiting in a long line to be seated for lunch. Check out any restaurant you suggest to make sure you'll be able to dine at the hour you prefer.
6. Drink to excess
The days of the three-martini business lunch are thankfully over. Do you really need a drink with your lunch?
7. Argue with your server about the food or the service
If your food was cold or the service was bad, so what? Don't make a pleasant lunch date unpleasant by arguing about it. Just let it go.
8. Make or accept a lunch invitation without clarifying the financial arrangements
Never assume that someone who invites you to lunch will be paying for your lunch. An easy way to determine who's paying is to reply to a lunch invitation with "You know, I'm watching my budget but perhaps we can have coffee one day." If your friend intends to pay for your lunch, they will reply and tell you so.
9. Cancel a lunch date at the last minute except in a true emergency
A true emergency is when you, a dear friend or a close family member is physically ill or injured. That's it. If you cancel a lunch date at the last minute because you're too busy at work, then you don't deserve any more lunch dates.
10. Order a dish that will inconvenience your friend
People are busy. They don't want to have to sit with you while the chef prepares the house paella or any time-consuming dish for you.
11. Take a phone call, answer texts or fiddle with your phone during lunch
The minute you get to the restaurant, put your phone face-down on the table, or put it in your pocket. You are at lunch with someone who deserves your full attention. Leave your phone alone!
12. Let your subordinate pay for their own lunch
If you are a manager and you invite someone on your team to have lunch with you, you pay for their lunch -- no exceptions. The company pays for your subordinate's lunch if the policy allows it, and you personally pay for it if the policy doesn't allow. You asked someone on your team to give up their private lunch time to hang out with you, so you foot their lunch bill.
13. Talk about other employees
It is astonishing what people will say aloud in a crowded restaurant. Don't make your lunch partner or other diners uncomfortable by talking about private company matters at lunch, no matter how much you insist that nobody else cares. It's impolite and tacky to talk about people behind their backs, and doubly so in public.
14. Bushwhack someone
Many a struggling executive has been taken to lunch by their CEO only to be told "We're going to have to let you go" right over their dish of pasta. Bushwhacking someone at lunch is a horrible management technique. It's a technique used by cowards who need to deliver bad news in a public place so the recipient of the bad news can't react to it the way they would in a private place, like the office.
15. Fail to say "Thank you!" when someone buys you lunch
The fact that good manners require the boss or the person who made the lunch invitation to pay the bill does not mean that the person whose lunch was bought for them doesn't need to say "Thanks for that wonderful lunch!" They do. Even if the lunch was atrocious and the service abysmal, you still have to say "Thanks!" After all, it's the thought that counts.