I just recently started using Blender, so I am new to this.
Currently I'm working on a model of a mask. For that I created a flat rectangular shaped block, where I'm trying to cut out eye holes using the boolean modifier and a cylinder.
After applying the boolean modifier I fixed the ngons and removed doubles. Now I get the unwanted result of a face overlapping the edges of my mesh:
As you can see the marked area creates an overlapping face. In the render it will look like a rotated square:
I appreciate all the help.