What if I forget to feed it? When sourdough starter is neglected — left on the counter or in the refrigerator for extended periods without being fed — the yeast and bacteria run out of food, and their populations gradually decline. Basically, your starter is dying a slow death; the longer it's left unfed, the more difficult it will be to revive it and get it ready for baking. Feeding your starter regularly can prevent this situation. And also cure it: If you do happen to forget your starter for a while, a series of regular feedings should restore its health.
What's the "right" aroma? Sourdough starter should smell sharply sour but "clean," with no overtones of spoilage, or any oddly chemical smell (think acetone). While the aroma of sourdough may make you draw back simply from its strength, it shouldn't be distasteful.
When to throw it out? When your starter is neglected for an extended period, the liquid on top tends to turn from clear to dark-colored. That's OK; simply stir the liquid back into the starter and feed it the meal it's desperately craving. However, if your starter shows visible signs of mold, or an orange or pink tint/streak, or smells — well, putrid — throw it out; it's been invaded by harmful microorganisms, and it's time to start over.