Nigeria
Proper greetings are a MUST when it comes to elders. A 'Good afternoon 'ma'/'sir'' should suffice. Simply saying 'Hi' to an elderly person abominable and will tarnish your reputaion.
When in an open-air market; learn to speak 'pigeon' (a local version of english that most non-native speakers can understand). Alternatively, travel with a local as foregn accents will only attract higher prices!
The market place is not a sight-seeing area. For your own peace, do not go if you have no intention of actually buying anything, you may be berated if you play with the market sellers time!
When eating a meal, reserve your meat till the very end. This is often a subtle test your self-control: how well can you delay gratification? You will be judged!
There is a cultural stigma about shaking hands and passing items to adults with your left hand. If in doubt, best bets are to use both hands.
When crossing the road - outside lagos and Abuja cities - run when you have the chance to as road signals are not always obeyed! You may be waiting a long time to cross the road if you expect for cars to wait for you!