How to reproduce:
1. Create a new empty project
2. In the Hierarcy window click on MainCamera
3. In the inspector, select the color picker next to "Background" and select any color
4. Hit the Play button
5. Notice errors:

rc.right == m_GfxWindow->GetWidth() && rc.bottom == m_GfxWindow->GetHeight()

GUI Window tries to begin rendering while something else has not finished rendering! Either you have a recursive OnGUI rendering, or previous OnGUI did not clean up properly.

Not reproducible: 5.3.5f1