I'll try to show you few most common issues that happen with normalmaps, explain why they happen, and how to prevent them, so you can focus more. I'll use 3ds max and marmoset, but problem fixing will be mesh-dependent, so you can use whatever software you're using.
There are few other cases/issues to cover, but not everything at once - I'll try to make more materials as long people show up with more issues.
