Prob: A thin line appears when using blending

When I use blending a thin black line appears diagonal right across the window that I want to blend.
Here is a picture that show what the problem is.
The picture shows a window that is half transparent. The glass is a GL_QUADS and is defined by four vertices.
Why does the line appear and how can I fix it?

Well, internally the video card driver might divide your GL_QUADS call into triangles in order to optimize its rendering.

my guess is that what you’re seeing there is the quad broken into two triangles and then the texture border is being sampled due to non-point sampled texturing.

try setting your texturing to GL_CLAMP_TO_EDGE and see if that fixes the problem.

To turn of GL_POLYGON_SMOOTH made the thin lines disappear.