A list of embarrasing screw-ups I tend to waste time on (repeatedly) while writing OpenGL based rendering code. The list will surely grow as I continue working on Twiggy :P
This is generally stuff which doesn't show up with glGetError() of course.

Sampling a texture just returns black pixels in the fragment shader:

Most likely the texture is incomplete because the texture sampler states GL_TEXTURE_WRAP_S, GL_TEXTURE_WRAP_T, GL_TEXTURE_MIN_FILTER, GL_TEXTURE_MAG_FILTER hadn't been setup yet, or the FILTER states expects mipmaps but the texture has none.
Also on some platforms / drivers it is more efficient to setup those states before calling glTexImage2D().

Clearing a render target doesn't work as expected (eg the color buffer is cleared, but the depth buffer isn't):

glClear() is affected by the state of glColorMask(), glDepthMask(), and glStencilMask(). If glDepthMask() is set to GL_FALSE, the depth buffer won't be cleared even though glClear() is called with the GL_DEPTH_BUFFER_BIT set.

Also: http://www.opengl.org/wiki/Common_Mistakes