If you add a source folder with errors (or create a new one, eg "src2", add a scala class and cause a compiler error), and then right click and do "Build Path > Remove from Build Path", the errors remain in the problems view.

If you try to run a class with a main method, it will give you the warning that there are errors in the project.

This problem persists if you close and re-open the project, but goes away if you clean the project.