What are some unwritten social rules everyone should know ?

Try not to fart in closed premises like meeting rooms, elevators, vehicles etc. if there are other people around.

While using public transport, avoid asking those seated 'thoda adjust karo bhai' (please adjust a bit). Seats are designed for a specific number of people. Three people trying to sit on a seat designed for two, will make the journey painful for all the three.

If you are sitting on an aisle seat in a flight, don't lean towards the passage to rub your shoulders against the cabin crew members.
When you are in a multilingual group, use a language that everyone in the group understands.

When you make a phone call to someone who does not know you, start the conversation by introducing yourself instead of Hello.. Hello...Hello....Hello.

Don't be rude to telemarketing executives. Politely say 'No' if you are not interested in what they are trying to sell. You have the option of not answering such calls with the help of applications like True Caller.

To Telemarketing Executives: if the customer refuses the credit card or insurance policy you are trying to sell, don't keep on asking WHY. You can still try to convince him.

Leave people from workplace on time: You may be a workaholic. You are free to be one. But, don't call for meetings at 5.00 and make people sit impatiently. Such meetings are not
productive at all. Leave people free by 5. You may continue beyond with those things you can do on your own.

When you opt for car pool, book it only after you are ready to leave. And, do not expect the car to pick you up from your doorstep. You may walk to the main lane/road nearby and save the time of your fellow passengers.

While car-pooling, it is not necessary to occupy the backseat that is diagonally opposite to the driver to establish your importance. You should be willing to move to the other side for other passengers to get in.

When a colleague is leaving the organization, don't ask 'where are you joining?'
Being veg or non-veg is purely an individual choice. Don't ask anyone why it's so.

When a friend/colleague offers you some food, you can politely say No. But, don't do this after tasting or smelling it. It's an insult to the one who has offered it to you.

Eating with hands is perfectly fine. Don't stare at someone for doing it as if s/he is a barbarian.
When someone starts talking about their ailments, don't start talking about yours