Face Overlapping Edge 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.
As previously said by Carlo: "Concave polygons can cause artifacts" because Blender is not sure how he is supposed to fill the surface between the vertices.

So here is a better way to make a hole in a 4x4 surface. I think you should avoid Booleans as much as possible, especially for such a simple operation. To make a hole you can simply select the central point and delete it. Then to round the shape, select the edge loop and use the LoopTools addon > Circle. Then you can extrude once, then scale down the extrusion, give it the shape you want, etc...