Density of water
It seems that a lot of people mistake the density of water for 1 oz./fl. oz.. While it's close to that, it's not quite right, and sometimes people do mistake the density of water and weigh out, say 7 oz. of water when the recipe calls for 7 fl. oz. of water. In imperial system, the density of water is actually 0.9587 oz./fl. oz (moderator's note: see this comment below for disagreement about this number). In metric, water is conveniently 1g/ml (when it's pure, and tap water is close enough to pure for that to be the density) at room temperature. And in case you don't believe me, don't be afraid to bust out your scale and confirm this yourself. Measure out whatever volume of water in ml, and the weight will be the same in grams. However, measure out a certain volume of water in fl. oz., and you'll find that the weight in oz. won't be the same (or weigh it out then measure by volume if you want to be a little more precise).