Most people think they have a good grasp of how roofs work – the shingles (usually asphalt) keep the water out, and that they do that by being the waterproof layer that stops water from getting in. That’s more myth than fact.
Another myth, is that caulking is an acceptable way to achieve waterproofing.
Another myth, is that if you have ice-and-water shield on your roof, you’re good.
What all these myths have in common is that they misunderstand how a sloped roof system works in reality.