How to avoid causing offence

1. Delay.
This gives you time to think (of the effect of your words) before you speak. Use a niceness filter.

2. Never attack a weak-point.
Every person has a shortcoming that he/she is aware of. Never hit the chink of their armour. For example never tease a short guy about his height or a dull girl about her intelligence.

3. Let go of your ego.
If you stop caring about your own "importance" then you will hardly pay attention to any "affronts" to you - real or imagined. Then there will be no need to retaliate. This is actually the secret to never having quarrels/fights: Not minding.

1 is hard to do and requires a lot of practice.
2 requires that you know the person well.
3 is hard even to accept; it is so contrary to what we hold on tightly to. But imbibing it will make you a much happier and friendlier person.

College days

This incident happened when we had just joined college. The computer science HOD had changed. The new professor was really thin and short. It was a commonly held belief that no matter what he wore, he'd always look like a peon.

Anyways, so a computer lab class was just over and a first year student from ECE had left his pen among the computers somewhere. He somehow got the impression (probably from his looks) that our new HOD was one of the lab boys. Here's what followed:

Student: "Mera pen kho gaya hai"

HOD: "Haan, toh dhoondh lo na"

Student: "Aap bhi meri madat karo na"

HOD: "Nahi nahi, tum khud hi dhoond lo"

Student: "Kya bhaiyya, aap din bhar bas baithe rehte ho. Kuch karte-warte toh ho nahi...."

Imagine telling the head of the department that he's good-for-nothing!

HOD(SHOUTING): "I AM THE HOD OF THE COMPUTER SCIENCE DEPARTMENT !!! HOW DARE YOU SPEAK TO ME LIKE THAT...."

Poor guy got a good dressing down. And we just couldn't stop laughing when we heard the story...