Some people consider Arabian Nights to be a 78-card set, since there are 78 unique card names in the set. But many collectors view the set as actually having 92 cards, since there are printing differences on several of the commons' mana symbols, resulting in two distinct versions for each.
Eleven of the twelve commons that have generic mana in their mana costs appear in two versions, with Desert Nomads being the only exception. An example is shown below.
Note that one version has a small, dark generic mana symbol, and one has a "normal looking" symbol.
In addition, three other commons--Hasran Ogress, Nafs Asp, and Stone-Throwing Devils--were printed with slightly different colored mana symbols; these differences are very subtle.
The cards with smaller, darker symbols are known as the "a" variations, and the normal-looking cards are known as the "b" variations.
The rarities of each: (Please note that our clickable card links do not correspond to the different variations.)