Unlike, for example, almost anything that is made on the stove and even cakes (of course, as long as your cake doesn't fall as you take it out of the oven), if you stop baking a loaf of bread for some reason before it's thoroughly cooked it will never lose that raw taste, no matter how much you bake or toast it later.  Why is this so? Is it a property of bread? Is it something to do with gluten? 
Hot gasses such as steam and CO2 trapped inside the dough by the crust are important to help properly bake the bread, as well as to give it form and structure. If you cut a slice off the end of the bread before the bread has finished baking, you completely change the conditions under which the bread finishes baking: steam will escape rather than building up, the internal temperature probably won't rise as much, and the bread will tend to dry out rather than cook.  If you're not sure whether your bread is done, you should take its temperature. An instant-read, digital thermometer with as fine a probe as possible is best because it compromises the crust the least.  For the same reasons, you should let bread cool as @justkt suggests before cutting into the loaf. Bread smells great when it's hot, but it tastes best when it has cooled somewhat. 