Eclipse C/C++ Shows Errors but Compiles?

Question

So I am building some Arduino code in eclipse, as described in Your Second Arduino Project, but every time I use an Arduino library, such as Serial, Eclipse und

Answer 1

I had include folders in

Project Properties -> C/C++ General -> Paths and Symbols -> Includes

When I removed those, the red underlines went away, i.e. the build and the IDE where in sync.