OpenGL: glScale(1,1,1) + lighting screws up quad rendering!?

#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <stdio.h>
#include <stdlib.h>

void draw(void)
{
    GLenum e;

    glClear(GL_COLOR_BUFFER_BIT);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glTranslatef(0, 0, -5);

    /* no problem without call to scale */
    if (1) glScalef(1, 1, 1);
    glColor3f(1, 1, 1);
    glBegin(GL_QUADS);
    glNormal3f(0, 0, 1);
    glVertex2f(-.5, .5);
    glVertex2f(-.5, -.5);
    glVertex2f(.5, -.5);
    glVertex2f(.5, .5);
    glEnd();

    glFinish();
    e = glGetError();
    if (e != GL_NO_ERROR) {
        fprintf(stderr, "OpenGL: %s\n", gluErrorString(e));
        exit(1);
    }
}

#define W 400
#define H 400
void setup_gl(void)
{
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glViewport(0, 0, W, H);
    gluPerspective(40.0, (GLfloat)W/(GLfloat)H, 0.1, 1000.0);

    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);

    glShadeModel(GL_FLAT);
}


/* toolkit specific */

static void display(void)
{
    draw();
    glutSwapBuffers();
}

int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_DEPTH);
    glutInitWindowSize(W, H);
    glutCreateWindow ("gl window");


    glutDisplayFunc(&display);
    setup_gl();

    glutMainLoop();
    return 0;
}