We don't like to admit this to ourselves, but occasionally we'll take advantage of our own children's gullibility. Sometimes, a well-meaning lie is better than trying to explain a concept idea or trying to have an awkward conversation. Anybody whose child has asked them where babies come from before the age where it's appropriate to tell them will know exactly what we mean by this.
It isn't always just a case of lying to spare our own embarrassment though. Sometimes we lie because it's easier than having an argument. Trying to explain to a young child that having McDonald's every day isn't good for them is asking for a fight you can't win. Just have them believe that it doesn't open every day of the week until they're old enough to find out for themselves. It's better for everyone that way!