The classic "find me a rock" story is as follows: A manager goes to his engineer one day and asks for a rock. "A rock?" asks the engineer. "Yes, a rock. That isn't going to be a problem, is it?" replies the manager. The engineer laughs and tells the manager he'll go pick one up during his lunch break and it will be no problem. After lunch, the manager visits the engineer again and the engineer shows him the rock. The manager looks at it for a moment before telling the engineer, "No, that one won't work at all. I need a rock."
"Find me a rock" problems sound dead simple, but in actuality have requirements that are poorly stated or unknown. You never know what you're looking for; you only know that you'll know it when you see it.