A friend of mine was backpacking in the mountains. Early one morning, he was hiking a trail that lead to a stream. The scene was breathtaking. Snow capped mountains overlooked the stream. The only sounds were birdsongs and the flowing stream. No sign of human touch cluttered his view. The clean outdoors extended before him as far as he could see.
Overcome with the beauty of nature, he stooped to drink from the stream. The water was the most disgusting thing he had ever tasted. He looked upstream and saw a dead mammal decaying in the stream. The carcass was upwind so he could not smell it yet. He decided to stick with his canteen after that.
On an actuarial exam, make sure that you understand the information given in the problem and how it applies. Exam questions provide enough information to do the problem in several incorrect ways. With practice, you can identify the misdirection. Otherwise, you may drink some nasty water.