This is a list of common mistakes made by OpenGL programmers.
1. Texture Upload
You create a texture and upload the pixels with glTexImage2D (or glTexImage1D, glTexImage3D) but there seems to be diagonal lines going through the image or your program crashes. This is because the scanline of your pixel array is not multiple of 4. The scanline is width * bytes. By default, glPixelStorei(GL_UNPACK_ALIGNMENT, 4) and you can change it to glPixelStorei(GL_UNPACK_ALIGNMENT, 1) if you scanline is not multiple of 4.
2. Texture Precision
You call glTexImage2D(GL_TEXTURE_2D, 0, X, width, height, 0, format, type, pixels) and you set X to 1, 2, 3, 4. You shouldn't set X to a number because although your code will work, it's not really an internal format. You should set it to GL_RGBA8 or some other "internal precision" format. The GL documents have a table of valid values.
3. Depth Testing Doesn't Work
You probably did not ask for a depth buffer. If you are using GLUT, glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH | GLUT_STENCIL) GLUT_DEPTH asks for a depth buffer. Be sure to enable the depth testing with glEnable(GL_DEPTH_TEST) and call glDepthFunc(GL_LEQUAL).
4. No Alpha in the Framebuffer
Be sure you create a double buffered context and make sure you ask for a alpha component. With GLUT, you can call glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH | GLUT_STENCIL) in which GL_RGBA asks for a alpha component.