For Americans, binge drinking and massive coffee intake are probably at the top of the list.
For the French, smoking and mistresses in a back closet.
For Djiboutians, its khat and cha.
Khat is a narcotic similar to chewing tobacco. But in my personal opinion it takes gross to a whole new level, as the man chewing does not put a small wad in his lip, he puts a HUGE wad in his cheek. If you have ever sat next to a man dipping, you know that the spit is coming, so you anticipate the nastiness. But multiply that nastiness by a factor of 25. A khat chewing man does not require a small coke can, he requires the entire road to contain his spit (or rather to let it be free). Gross.
Here is a picture of one of the khat vendor:

Cha is simply the word for tea. I partake of this beverage each morning which includes black tea with a cup of sugar for 8 cups, milk for tea (aka condensed milk), and spices of your choice (cardamom, cloves, cinnamon or ginger). Quite tasty, however it will probably give you at the least a few extra pounds and at the worst cavities and diabetes. Yet, as with all "vices", I ignore the consequences and drink myself to sweet bliss.