Some common mistakes about diplomatic protocol and how to avoid them:

Diplomatic service is sometimes mistakenly called "Foreign Services", instead of the correct, "Foreign Service". It is only a single service in a larger group of civil "services".

Diplomatic Corps is the body of all foreign diplomats accredited to a particular country. The CD written on number plates of diplomatic vehicles is an abbreviation of Corps Diplomatique (French for Diplomatic Corps).

The Corps here is pronounced as "kor" not "korps".

In conversations, the correct way to address a foreign Ambassador is either "Mr./Madam Ambassador" or "Ambassador + surname*".

*If a female Ambassador keeps her maiden name after marriage, then do not use her husband's surname to address her.

It is not correct to address an Ambassador's wife as "Mrs./Madam Ambassador". She is addressed as "Mrs. + surname*". Similarly, husband of an Ambassador is addressed "Mr. + surname".

*if she has kept her maiden name after marriage, then use "Ms. + maiden name".

"Excellency" is commonly used to address a foreign Ambassador. Avoid using "Excellency" for diplomats that are lower in rank to Ambassador. Also avoid using "Excellency" for Ambassador of your own country.

Some diplomats (who are not of Ambassador rank) may even disapprove being called "Excellency". They may even correct it, by saying "Sorry, but I am not the Ambassador". So be careful in formal usage of the word, your gesture of respect may be construed as "sarcasm" or "flattery".

In written communication, an Ambassador is addressed in the following manner:
"His/Her Excellency (or H.E.) + Mr./Madam + first name + surname, Ambassador of (foreign country's name) to (your country's name)"
For example, "H.E. Mr. Nong Rong, Ambassador of China to Pakistan".

The use of the honorific title of Ambassador is prevalent, especially among career diplomats, when they are no longer posted as Ambassadors. Usually former Ambassadors use "Ambassador, retired" to avoid confusion. It is polite to address them "Ambassador + Surname".

Usually in every diplomatic mission, both career and non-career diplomats are posted. Career diplomats belong to the "foreign/diplomatic service" and Foreign Ministry. Non-career diplomats belong to other ministries, and may belong to diverse professions other than diplomacy.

There is a quota for appointing non-career Ambassadors to Embassies abroad. Except, for exceptionally brilliant individuals, generally non-career Ambassadors do not receive the same level of appreciation, respect and attention, as those Ambassadors, who are career diplomats, do.

How to spot a non-career diplomat/ Ambassador?

Ask them about their previous foreign postings*. Usually they wouldn't lie & immediately tell you that they are not career diplomats. Once spotted, communication becomes easier.
*Non-career diplomats normally get one posting abroad

Always use the exact name of an Embassy to avoid confusing it with another Embassy. E.g, "Pakistan's Embassy in Greece" is different from "Greece's Embassy in Pakistan" - the former is a Pakistani mission located in Greece, the latter is Greece's mission in Pakistan.

It is mistakenly believed that "diplomats cannot avoid social drinking". While social drinking is considered important in diplomatic circles, it is neither compulsory nor is abstinence from drinking looked down upon. Social drinking is but one of the numerous shades of diplomacy!

High Commissioner and Ambassador have the same diplomatic rank. High Commissioner is used for Ambassadors of Commonwealth countries who are accredited to another Commonwealth country. E.g. Australian Ambassador in Pakistan is called High Commissioner of Australia.

A Country Representative (formal: resident representative) is accredited by an international organization (generally a United Nations agency) to a country's government. A Country representative is below the rank of Ambassador.

A Permanent Representative is a diplomat, usually of Ambassador rank, who is the head of a country's diplomatic mission to an international organization, especially UN. This is why Ambassadors in Geneva and New York are called Permanent Representatives.

Some people mistakenly believe that a Permanent Representative (PR) is a permanent Ambassador (and isn't changed until he retires or passes away or chooses to quit). The word "permanent" refers to the "Permanent Mission", which is headed by the PR.

Hierarchy in a diplomatic mission is usually as follows:

Ambassador
Ministers
Minister-Counsellors
Counsellors /Military Attachés
Civilian Attachés (not in the Foreign Service)
First Secretaries
Second Secretaries
Third Secretaries
Consular Attachés (May or may not be diplomats)

Hierarchy in the Ministry of Foreign Affairs or Foreign Office is as follows:

Foreign Secretary / Secretary General
Special Secretary
Additional Secretary
Director General
Director
Deputy Director
Assistant Director