OpenGL 2D hud in 3D application

Question

I have been trying to create a hud in my OpenGL application. Having looked around, it seems the way to do it is with an ortho projection, but so far I have not been able to get the program to render correctly. What is happening is instead of rendering on top of my display, I'm getting odd graphical glitches as seen here:

If I comment out the hud code, everything renders perfectly.

glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LESS);

//Set up projection matrix
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//Using gluPerspective. It's pretty easy and looks nice.
gluPerspective(fov, aspect, zNear, zFar);

//Set up modelview matrix
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

//3D rendering

glDepthMask(GL_FALSE);
glDisable(GL_DEPTH_TEST);

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0,window_width,0,window_height); //left,right,bottom,top

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glColor3f(0.0,0.0,1.0);
glBegin(GL_QUADS);
    glVertex2f(50,50);
    glVertex2f(50,100);
    glVertex2f(100,100);
    glVertex2f(100,50);
glEnd();

Answer 1

Once you're done rendering the HUD, you need to re-enable the depth writes

glDepthMask(GL_TRUE);

Answer 2

When you clear the buffers when rendering your HUD, all that has been drawn so far (your 3D scene) will also be cleared. So don't clear the buffer twice.